4-cyl, or v8?

[Unit 5302]

If you are going to be swapping out the engine anyway, then get the 4 banger. A new rear end won't set you back anywhere near the premium on a 5.0. Plus you shouldn't really be using the old 5.0's tranny in a 351. The extra torque load will be too much for it more than likely. Sounds like you plan on making it pretty powerful, and the T-5 won't be around long on a 400hp 351.