[clay68c10]

Looks like a great working-man sort of project. I like it!
This truck will be a good one for you to wrench on if you were doing that sort of thing back in the 80s. They are dead simple and pretty forgiving.
Verify that you don't have a shift linkage problem causing your high RPMs. If the trans is worn out, have that TH400 gone thru and you will never touch it again, except for fluid changes. They are one of the best automatic transmissions ever made.
As said above, the 402 is common enough. Verify that you are using oil before you pull the trigger on any major repairs. How do the plugs look? Do a compression and leakdown test. If it's just running rich, it will blacken up the exhaust.