Joined: Nov 2016
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.
I smile a lot. It makes people wonder what I'm up to...