As you say the individual parts are all the same difference lol.With the exception of the pistons, every part is interchangable. Like the posts above state, they may require modification.Besides the steam holes on a 261 head(#850) the 261 head supposedly has larger combustion chambers.My observation is ,GM lowered the compression by lowering the piston height at TDC.Perhaps someone has actually CC'd the combustion chambers?The #848 head is the only 235 replacment offered by GM in later years, but I have seen a few other casting numbers on 55-56 engines. 261 rods fit properly on a 235 crank, but the wrist pin holes are larger ,so custom pistons are required.The crank shafts have the same part number as do the valve springs, but 261's use rotaters on the exhaust valves.The water pump impellers and bolt pattern are identical but the castings have different fittings and so on. The intake on engines with governors is a lower profile .My old GM parts books list the same part numbers for most every thing except the differences mentioned. Original manafacturer replacement parts are often standarized more than the original stuff to lower dealer inventory costs.
