I have a th400 or possibly th475 and i asked more or less the same question and the 4L80e is apparently what these evolved to be. So that's what I'm going to down in the future.
I'll have to try that for the front seats, rewebbing.
Back seats I'm upgrading from 2 point to 3 point and going with
https://www.seatbeltplanet.com/i-30499897-1989-1991-chevy-pickup-truck-crew-cab-rear-bench-seat-belt-kit.html?q=500268
How did the hvac move up go? Seems the wires are long enough.
I'm going to do the same thing and possibly switch it do a double din down there.
I have the stock radio that works (turns on etc but no speakers connected to it so idk, plan is testing it) I'll be sure to offer it up when that...
My ps squeals at full lock i thought that was just because it's old af lol.
It could be bc it doesn't have atf in it?? Any other possible cause? Fluid is at proper level, I'll take a look at the cap and see what it says.