You say you know LS engines... Have you considered the 8.1L big block? You can get a complete truck for $1500 if you're lucky. It will bolt to the TH400 and still has the bosses on the block for traditional SBC/BBC mounts like your truck. You can use your stock exhaust manifolds or get headers...
When I replaced all the channels in my 90' Suburban with power windows I used Honda Shin-Etsu grease in the channels. It's expensive and will make a mess of the glass if you're not careful/strategic, but it holds up really well. Between the grease and the window relay mod with auto-down they...
Reviving a dead thread to give the part numbers from AuroraGirl's post. The window regulator rollers are available from truckandcarshop.com, page 17 of the squarebody catalogue.
1973 - pn 73-31308 -$4.00ea
1974-87 - pn 74-31309 - $4.00ea (I'd expect they will fit up through 1991 as well)
If...