Unless the 235's are the later full pressure oiling, they are not going to be any better of an upgrade over the 216 engine. All the 261's are the more desireable full pressure oiling and a much better upgrade even over a 235, so you would be gaining a better engine with the 261. As far as being rare, they were mostly a truck engine here in the US, and production #'s might not have been that high compared to passenger cars, but they aren't too hard to find.
