From the 1961 235:
You take off the clutch assembly.
Then, you take off the flywheel.
Then, you take off the clutch housing (bellhousing).
You might want to put into the 235 a new pilot bushing.
You then put onto the 1961 235 engine:
The 216's clutch housing.
Then, you put on whichever flywheel suits your tastes (6v 216's vs 12v 235s)
Then you put on the clutch assembly that matches up to the flywheel.
Be sure to use the throw-out bearing that matches the clutch assembly.
