It's impossible to get a 3 3/4" bore into a 235 block without hitting water, or at least getting so close the cylinder walls will fracture the first time the engine is run. The block core is different on the 261, with the cylinders "siamesed" together like the small block 400 V8. That's why the extra cooling passages are in the top of the block and the cylinder head, to eliminate hot spots when the coolant can't flow between cylinders.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!