be hard to find a flywheel for 460 and would be high. If the 351M/400 bolts on it i could have it balanced to fit the 460.

 

IMO, rebuild the 300 w/offy dp intake and Holley 390 4bbl(what you wanted, lol) or E-brock 500, or motorcraft 2150 2bbl. EFI or other headers, and either lower the gears, or go to a smaller than 33" tire. Or 460, which from a 300 truck is the most expensive swap(a 351M/400 truck would have been alot cheaper to put a 460 in.). Anything else is not going to produce low end torque for pulling/towing. Better for highway, yeah, but not for low end. Now the 460 flywheel can be found from aftermarket places pretty easily. With all those other motors, I suggest some old restoration projects. my 2 cents, and good luck, whatever you decide.

 

The 460 and 351m - 400 use the same bellhousing. A transmission from a 460 will bolt right up to a 351m/400 and vice versa. However, they do not share flywheels - they are different. Personally, all the motors that you listed are very good choices, but the 460 is going to give you the most torque, have the strongest bottom end and best oiling system, and give the most power without really building it up very much. I have a 74 460 that I put trickflow aluminum heads on, with a matching cam, intake manifold, and 10:1 pistons that puts out at least 500 (conservative) hp in my 78 f-250 4x4. I built it myself and it cost about $2,500 (roughly) to build. The heads were the most important and expensive part of this endevor at about 1600 bucks. It idles well but with a slight lope, and runs like a dream come true. If you are looking to pull you would want to go with a more mild cam, but you would still be in the 450-500 hp range very easily with this setup with more torque than you will know what to do with. In fact, you better set up an account at the local tire store - cause you're gonna need new ones on the rear often. Also, you will probably need a plastic surgeon to remove the smile off your face. However, I will say, you can do something very similar with the 400 as well. They are all pretty good choices, you just need to decide what you want your truck to do.

 

Also, 460 flywheels are not that hard to find. Most wrecking yards are going to have a few for you to choose from. You can even get them off 460 Lincolns from the late 60s through the 70s. I wouldn't let the flywheel stand in the way of any conversion. However, if you are bound and determined to stick with the 4 speed, they may be a little more difficult to come by. But, I bet schucks and napa carry them - as well as clutch manufactures.

 

I have heard that a 390 flywheel will fit the 460.

 

I have heard that, too.

 

And that may be why the wrecking yards have them all the time. No shortage of those.