Hy 1954Chevy5Window, By swapping the front mount plate from the 216 to the 261 and using the 216 bellhousing, it is almost a dropin swap. The other issue to deal with is the water pump, post 55 235s and 261s have a longer water pump than the old 216s, and the newer pumps sit lower in the engine than the old ones. There are a couple of different ways to deal with this, do a search on water pumps I'm sure something will be there, hope that helps, and good luck, and lastly keep us informed.
