[CC69Rat]

Late 70 through 72 will be a direct swap for the rear end housing. It will be 57" long measured from the inside edge of one rear brake backing plate over to the other side in the same spot. (If you go that route)

Earlier rear end housings will be .75" more narrow on each side so it will impact your wheel backspacing a little. It will work, they're just a bit more narrow than 70-72.

But, personally I'd just have yours built. As others have mentioned I'd find another shop.

With your setup I'd put a shift kit in the trans and 3.73 posi rear. Gas mileage will be brutal with either 3.73 or 4.11 but .. 3.73 might help the wallet a little and still get your truck out of the hole a little quicker.

Do you do any freeway driving at all or ever plan to? Do you do any towing?