[SactoJim]

Many years ago I switched from a "3 on the tree" to a TH350. Things you need will be:

Auto crossmember
Drive shaft mods
Auto linkage
Trans cooler
kick down cable and carb brackets (400 similar but electric)
Automatic steering column (if not doing a floor mount type shifter)

Those are major ones that I can thing of at the moment.

TH400 is a good tranny and REALLY good if it's free, but after sitting that long you should at least put new seals in it. You certainly don't need it behind a mild small block and it does take more hp to operate than a 350 or 700.

I'd lean torwards the 700 because it would simply make your truck more fun to drive. The 700's lower first gear will get you off the line quicker and you will get the added benefit of an overdrive.