[jeffspower]

I've done several of them. I went the route you are thinking with an '80 K20- I took the Chevy TH400 4x4 output and put it in a Olds case. Mine was a '60s Olds case that didn't have a speedo drive, which is great because a 4x4 doesn't need it either. I's just another place to leak.

Where the firewall and floor flanges form a big lip on the drivers side, you will need to pound flat or cut it out. Maybe a small dent in the firewall on the passenger side for valve cover clearance. Fab up some mount perches and that's about it for custom work.