What year 261? Some of the suggestions below are different for different years.

-Water pump adapter plate - Yes, and a 1952 water pump

-It has a 12v generator so new regulator - Yes (and a ballast resister)

-Bellhousing? Do I use the 216's? - Yes (and, you might have to modify the front plate on the block for the front motor mount)

-flywheel? - use the 261's flywheel (12v starter?)
-Clutch? - use the 261's set

-Accelerator modification? - most likely. You should be able to use the bell-crank and up-rod to the carburetor; however, the pedal to bell-crank rod might have to be shortened.

Tim