318 poly wide block

If its a 4 speed and it is original equipment it should be an NP420. These had a 10 spline input shaft. You wont have any luck at the parts store for the clutch disk. I bought one at a place called "HR Clutch" for my powerwagon. If your bus has an 11" clutch like my truck then the standard pressure plate from a later model truck will work, but that thing might have a bigger one.