Someone might say you could morph a 235 into a 261, but it would not be a simple matter of increasing the cylinder bore. You would also have to add coolant passages (the 261 has 2 additional coolant passages between each pair of cylinders). Some people use the silly term "steam holes" for those additional coolant passages (GM called them coolant passages).
The 261 also had a cam with slightly modified lift (same lift as the cams in 235 Corvette engines). That style cam can be also used in any other high pressure 235s (and, I think it was used in later 235s).
