I just went through this with my 1968 302. Couple of bad head gaskets last opened up in 1983. The builder asked me what I wanted from the engine rebuild You can read about it and what I learned at my web page 1968 Shelby Cobra
. I told him I'd like to see 350 HP. That is a whole different path than staying "stock". He blueprinted and balanced the engine. Put in forged pistons, Edelbrock E-Street aluminum heads, Comp cam with a .509 intake lift, original Cobra Offenhauser aluminum intake, Tri-Y headers, and a new Holley Brawler 4 bbl with mechanical secondaries and dual gas feed. My car came with a 3.89 rear end. Toploader 4 speed. I ended up with a pretty cool 1968 Mustang. But my wife calls it "loud and obnoxious", which it is. Added about 100 horsepower to it.
1)My rear end is pretty low. At 65 MPH I'm running about 3200 RPM. It's even more noticeable with this engine. Unless you plan on doing a lot of drag racing, I'd go with a 3.50 rear end. Or a 5-speed. I'm pondering a 5 speed for the OD to get the highway RPM down. Be aware though if you go 5 speed that the T-5 comes in different flavors and some of them don't do well with 400+ HP.
2) I have a race car engine in my street car. Is that what you want? That is what I wanted. It's loud and real obvious it's got a big cam in it. My Mustang is set up for torque coming in at the lower end rather than high Horsepower at the top end. The heads, the Comp Cam, the Tri-Y headers, the Cobra intake all work towards good torque at the low end. 1500-5500 RPM is the band. How often do you max out the RPM? 5000 RPM is a lot of RPM. This setup will take you there quickly.
3) If the suspension and the braking are not up to par, you might want to start there. I replaced and upgraded both of those before doing the engine rebuild. You want to enjoy the engine and do it safely. Going fast is great but being able to stop the car is also great. It will be a lot more fun if the suspension is fresh and tight. Plus there are upgrades that makes it even better. If you don't have disc brakes, put that as number one on your list. Front discs for sure. Rear discs if you can swing the extra cost.
4) Before you choose a cam or heads, pick the path you want from that engine. A racing engine is one path, or you can go with a strong stock or some place in between. "Beast" says you want a racing engine. Read up on what you can do with a 351 W. Stock they are rated at 300+. How much HP would you like? 300 takes these parts, 400+ takes those parts. Find a good shop that has experience with old Ford engines with a carb. Most of the shops I talked to only work with EFI. Edelbrock says a good set of heads can add 35+HP. Choosing a cam depends on your path for this rebuild. Roller cams are a big upgrade in these old blocks. I'd be surprised if you could even find a shop that has done that. Extra cost over staying a flat tappet cam. In my build, a roller cam would have added about $500 to the valve train cost. My builder really wanted to stay flat tappet with roller tips. But if you want 400+ HP you need to go roller cam.
5) side effects: I promise if you give your car an extra 50-100 HP you will have a different car. And you are going to see flaws. I ended up with lower vacuum which now affects my power brakes. This cam shouldn't have done that but it did. My builder told me to get a new Holley with an electric choke. Sounds good. I do not have the choke worked out yet. None of these guys know how to tune a carb or an engine with a carb. The shop I got the carb from is going to tune my car next spring. I put Wilwood 4 piston discs on the front of my 68 Mustang so stopping is not an issue. And replaced all the front and rear suspension, dropping the car an inch on both ends. My car can handle the extra horsepower. Will yours, as it sits?
That's exciting. I just went through it, got my car back in August 2019. I documented what I learned on that web page. Click on "My GT350" and read what I did to my car. I added "why" I did those things. I did hours and hours of research on each part before I did the upgrades. Much of that info is on those pages.
Decide what you want your car to be. Make a plan and make it happen. If I had a 69-70 Sportsroof, I'd build a Boss 302, only better.