The flywheel, clutch, starter, and bellhousing from the 216 should be able to bolt onto the 261 block and crankshaft. The 1946 transmission- - - -is it a 3 or 4 speed? There was a major change in transmission design in around 1947/48, with a different bellhousing bolt pattern on the later transmissions. The earlier, lighter duty trans and clutch might be on the ragged edge, torque-wise, for handling the output of the 261, so don't abuse the light duty driveline components.
Jerry
