Any small Chevy block will do fine, I would stay with the automatic though. Get a turbo 350, put a shift kit in it, and you will be barking the tires between first and second. If swapping to manual, I would use a muncie, To swap trannies will require more work, such as the clutch, bell housing, flywheel, pedals.
If you dont buy the car, let me know I am interested.