Just looking at the engine, yes motor mounts will need to change. Either bought or homemade, you'll need to be able to do some welding before you're through.
What kind of budget are you on???
The other issue is the tranny. If you go with the automatic, you will need to remove the center crossmember and fab a new one farther back, and the rearend will need to be converted to an open driveline. Esentially, its the same amount of work as going to a V8 auto. setup.
Even if you stay with the original tranny/rearend combo, you will need to change the bellhousing, flywheel, clutch, linkage, etc... and still do something about that crossmember. I THINK the 63-66 trucks had a bellhousing with side mounts on it, which would help alot.
If we knew where you were from, we might be able to direct you to a source for a later 235 or 261...
