I6 250 swap to 351W Question - Ford Mustang Forum
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post #1 of 12 (permalink) Old 6 Days Ago Thread Starter
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I6 250 swap to 351W Question

I have a very nice 1969 coupe, original in-line 250 and my brother in-law just gave me an early 70's 351W
Mild thumping cam, aluminum heads, nice street motor but nothing overly aggresive.
The inline 250 is in front of the original manual transmission. Have no idea tranny model or anything but
I want to put the 351W in the car. I am good with changing the motor mounts, wiring, beefing up radiator etc.
but I am lost on what route to go with figuring out transmission setup.
It's my understanding the 351W is a small block ford and so i think I am looking for a bellhousing that bolts to a small block ford. Will the tranny on the I6 bolt right up to the 351W, even though it may not be ideal?
What would be the easiest route in figuring out a decent transmission to hook to it with the least modifications and cost.
I will stomp on it from time to time but mostly just ride around on a weekend.
Thanks for your help and yes, this is my first post

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post #2 of 12 (permalink) Old 6 Days Ago
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Welcome to AFM, Ecu!

I am guessing that this is the old toploader 3 speed? If so, you've got more than one issue at stake here. The internals on that transmission, while fine for a 250 i6, will probably not last well when exposed to the higher power output of your 351.

Going to a more modern 5 speed with overdrive would need a rear gears change, and the typical T5 won't be happy if you get rowdy either. I think if I were in your shoes, I'd try to find a 4 speed Toploader, which could take the power of your engine, won't require any rear gear changes, and can handle any type of driving.

There's only one other thing you will need to resolve, and it might change your options for transmission choice too: If you manage to hook your rear tires, your rear pumpkin's going to explode like a Halloween jack o' lantern in February, hit by a sledgehammer. It will be on borrowed time from the moment you put the 351's power to it.

In swapping your rear end, you're surely going to need to pick out gear ratios for it. And if you're going that route, you might as well go with 3.73's or 4.10's, and put in an overdrive transmission while you're at it. Given the 351's good low-end torque, I think I'd go with 3.73's.

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Originally Posted by Grimbrand View Post
Welcome to AFM, Ecu!

I am guessing that this is the old toploader 3 speed? If so, you've got more than one issue at stake here. The internals on that transmission, while fine for a 250 i6, will probably not last well when exposed to the higher power output of your 351.

Going to a more modern 5 speed with overdrive would need a rear gears change, and the typical T5 won't be happy if you get rowdy either. I think if I were in your shoes, I'd try to find a 4 speed Toploader, which could take the power of your engine, won't require any rear gear changes, and can handle any type of driving.

There's only one other thing you will need to resolve, and it might change your options for transmission choice too: If you manage to hook your rear tires, your rear pumpkin's going to explode like a Halloween jack o' lantern in February, hit by a sledgehammer. It will be on borrowed time from the moment you put the 351's power to it.

In swapping your rear end, you're surely going to need to pick out gear ratios for it. And if you're going that route, you might as well go with 3.73's or 4.10's, and put in an overdrive transmission while you're at it. Given the 351's good low-end torque, I think I'd go with 3.73's.
Good info and thanks for the reply. I will Google search "Ford 4 speed top loader" to get a better idea. What 'adapters" or anything would I need to do it after I find the 4 speed top loader? Also, would you just hit up craigslist and salvage yards for a Ford small block "4 speed top loader" then have it gone thru? Once I buy a small block Ford 4 speed top loader, will I need a different bellhousing, clutch, pressure plate, etc to mate it to the motor? Also will transmission cross member need to be changed or modified? This is the area I am lost in
Thanks
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post #4 of 12 (permalink) Old 6 Days Ago
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You just need to find a toploader with the standard 6-bolt "Windsor" pattern. Also common for them is the "big block" pattern bellhousing. If it came behind any 289, 302, or 351, it'll fit with your engine. Toploaders are not super common, but then again, an awful lot of people are upgrading to 5 and 6 speed transmissions, and tend to get rid of them. So, you'll find them for sale now and again. You will have to see if your block has the boss for the clutch linkage built in, or maybe consider a hydraulic setup. There's lots of ways to make it work. The factory Z-bar setup is fine, but sometimes it's not convenient for header fitment. Everything's a little more crowded in there than it was with the inline.


Honestly, I'm not sure, but I think your crossmember might be just fine. If it's not, the proper one should be easy to find.

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I'm assuming its the same 3 speed that 65-68 models came with behind a 289 or 302. It will hold up fine.

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Checking the Ford parts catalog I was surprised to learn that the smaller, 2.77 3-speed used for 6-cylinder engines was last used in a '66 Mustang. V8s didn't use the 2.77 which had no synchronizer on first gear. Starting in '67 both 6 and 8's used a 3.03 3-speed. Those numbers are supposedly the distance in inches between the shafts inside the transmission. For '69 Mustangs the 6s used a RAN 3.03 and the 302/351s used a RAT. (RAN and RAT were the first 3 characters of the tranny ID numbers, Ford 4-speeds in '69 were RUGs). That means there wasn't as much difference between 3-speeds for a '69 Mustang 6 vs. 8 as most people, myself included, might have previously thought. However, this link indicates there may be a small difference in length between a RAN and RAT.

https://classicbroncos.com/forums/sh...ad.php?t=24268
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post #7 of 12 (permalink) Old 5 Days Ago Thread Starter
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Originally Posted by Ivy66GT View Post
Checking the Ford parts catalog I was surprised to learn that the smaller, 2.77 3-speed used for 6-cylinder engines was last used in a '66 Mustang. V8s didn't use the 2.77 which had no synchronizer on first gear. Starting in '67 both 6 and 8's used a 3.03 3-speed. Those numbers are supposedly the distance in inches between the shafts inside the transmission. For '69 Mustangs the 6s used a RAN 3.03 and the 302/351s used a RAT. (RAN and RAT were the first 3 characters of the tranny ID numbers, Ford 4-speeds in '69 were RUGs). That means there wasn't as much difference between 3-speeds for a '69 Mustang 6 vs. 8 as most people, myself included, might have previously thought. However, this link indicates there may be a small difference in length between a RAN and RAT.

https://classicbroncos.com/forums/sh...ad.php?t=24268
More good info! This is why I came to this site.
Sounds like I need a RAT 3 speed top loader or a RUG 4 speed for the path of least resistance?
I see them on craigslist from $400 to $1500
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post #8 of 12 (permalink) Old 5 Days Ago
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No matter which you get though, if you have the 7.5 rear gears, change your axle or it will be RIP, with your 351.

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post #9 of 12 (permalink) Old 5 Days Ago Thread Starter
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No matter which you get though, if you have the 7.5 rear gears, change your axle or it will be RIP, with your 351.
understood
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No more go until you have stuff to stop!

(Upgrade your brakes.)
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Like others have implied, and in addition to the tranny, you will need to replace...
Front & rear suspension
Differential
Driveshaft/U-Joints
Brake System
Cooling System
IIRR, the steering components are the same 6 vs V8, but check that as well

Even with the engine given to you, it's a bit of a financial challenge to put this together and make it work correctly... not that it can't be done, but just know what you are getting into as it's not a find me a tranny and drop it in project.
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Last edited by Beechkid; 3 Days Ago at 06:10 PM.
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post #12 of 12 (permalink) Old 7 Hours Ago
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You will need to make sure that your existing bell housing fits the 351. Although a small block, it was about an inch or so taller than the 289s, so things like small block headers for a 289 would NOT fit the 351 small block.
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