I did this swap in a '50 pickup, and it is pretty easily reversible but there are a couple of issues you have to overcome:
1.) Water pump. I shortened the shaft of mine and used a Ford pulley. I don't have enough room to run a mechanical fan. If I had to do it again I would get the adaptor plate kit and run a 216 style pump.
2.) Throttle linkage. The bell crank pivot point on the 261 is not in the same spot as on the 216. I ended up shortening my pedal shaft. It also rubbed, so I flattened the curvature of the pedal a bit by squeezing it in a vise. Worked fine.
3.) Starter. If you want to keep the footstomp starter, you have to run the flywheel from the 216, get a 12V footstomp starter, or do something a bit more strange. I opted for the bit more strange- I wanted to use the 11" HD clutch on my 261 but I kept my 6V starter. I had the flywheel turned down at a machine shop to fit the 6V ring gear. It cost me $50 and worked great.
4.) Front mount. I did not have to modify mine. My 261 came out of a '62 Suburban and it already had the holes drilled in the front plate. You can either swap the front plate or just drill the holes in the one on your engine. If you don't want to mess with pulling both camshafts then just drill.
5.) Oil filter. You can't run the 261 oil filter bolted to the rear of the manifold without it hitting the steering column. I re-mounted mine to the front of the manifold by making new hoses.
