Manual tranny interchange possible?

[Michael A. K. G]

Hi folks.

I have a recently acquired '72 C-10 with a 3speed manual (Muncie, I think) and a column shift, behind a 250cid inline six. It has the integral low hump.

Is it feasible to replace this with a 4speed and regear with a lower (numerical) ratio in the diff, to approximate an overdrive? I know I'll have to cut a hole in the floor for the stick; my main question is, what about driveshafts and clearance. The one four speed I found in the junkyard was much more "boxy" than the 3speed I currently have; it has no tailshaft housing (and it was in a Chevy Van). Will it clear the low hump? Are there driveshaft issues?

This particular 4speed was also behind a 250cid inline six, but can I take a 4speed from behind a small block V8 instead? Or should I just take the small block with it (big sale on engines at this junkyard -- $70 including accessories and flywheel, assuming you can find one in one piece)? What would be a good displacement for mostly highway/mountain driving and light hauling (I don't need massive power, just decent, and ok gas mileage would be a plus).

TIA.