Switching from Standard to Automatic Transmission in 1990 chevy cheyenne 4.3 lt
Well, you're looking at a driveshaft change regardless.
A 90-ish truck would be a 700r4.
Off the top of my head shopping list:
Steering column for shift quadrant (swap it in whole)
Gauge cluster or shift indicator to bolt into standard cluster
Assorted shift cables
Transmission
Flex plate
Driveshaft
Maybe a new tv cable ( not sure if a b8 cable will work on a v6, length and adjustment is critical on the tv cable)
Some wiring rework.
Personally, I'd look for a donor truck and grab it all. That would be most cost effective and a straight bolt in.
Line 'em up side by side and start swapping parts.
Good thing is it doesn't have to be a v6 donor, a v8 transmission will bolt right up.
Torque converter might be a bit if an issue as the v6 probably has a touch higher stall speed the v6. Drivable, but a bit more of a dog.
If you want to swap in a th350 or th400 it's a bolt in, except you'll need a driveshaft again.
Then a few other custom bits like a shifter quadrant, flex plate, etc.
On other note is if you go with the 350/400 there's no overdrive so worse highway mpg.
Good luck with your swap.