What's a CMP?
Depending on the year of manufacture of the 261, the water pump will probably be mounted several inches lower on the front of the engine block, which creates problems with the position of the fan in most applications. There is a conversion plate available from one of the members here that allows the 216 style water pump to be fitted to the late model 235 and 261 blocks. The original equipment carb for the 261 was a Rochester with a larger venturi than the 216 or the 235. They are relatively scarce. The small bore 216 carb would starve the engine for air, not fuel, and run very rich. It would rob a lot of high-RPM power.
The distributor position is the same- - - -it might be necessary to swap the front timing cover plate from the 216 to the 261, or re-drill the later plate to accept the front motor mounts. Your 216 bell housing, starter, flywheel, and clutch should be a direct bolt-on swap.
Good luck!
Jerry
