Any companies selling complete small block overdrive transmission assembles?

Depends on the car as to how much hackery would have to happen. I remember an episode of one of the Saturday morning "drag a rust bucket out of the weeds and restore it in a 1/2 hour" type shows, they put a GV od into a 69-70ish road runner with a big block and 727, I don't remember any floorboard hackery beyond a few strokes of a sledgehammer