I swapped the 216 to a 62 261 in my '50 pickup. The mount points are the same. The things you have to concern yourself with are the length of the water pump and the position of the bell crank for the throttle linkage. I shortened my stock water pump and used a pulley with less dish that I found in a wrecking yard to fit. I can't fit a mechanical fan behind my radiator because it would hit the lower radiator hose. If you get an adaptor plate you can mount the pump more like the 216 and use a mechanical fan.
For the throttle linkage I just bent the stock gas pedal rod and shortened it a bit. The pivot point is in a differant place on later blocks.
If you are swapping a 235 to a 261 all you have to do is take one out and put the other in. Just use the water pump setup from the 235 on the 261.
