You need a 10 bolt eaton from a 64-72. There are still some available with 4.10 gears, they're getting harder to locate. The swap is easy.
I would upgrade the brakes. You should be able to get some Bendix from a little newer truck and swap out the backing plate and all. Far more efficient than the Hucks. Also, look into different material for the shoes. If you search this forum, you should be able to locate an outfit in Texas that does kevlar linings. Not cheap, but it is supposed to make a huge difference in stopping.
Another option to condider is to put in a 700R4 instead of the TH350. Properly built it will hold up under the pressure and give you overdrive.
Biggest problem that I know of with a v8 in these trucks is having to move the steering gear to clear the exhaust.
