261 pistons have larger wrist pins than a 235, .060 bigger in diameter.So if you could put in 261 pistons you have to bore out the small rod end, or use 261 rods,the big ends are the same diameter.Looking at a 235 rod, there ain't much on the small end, never mind removing material.
Instant ID on a naked 261 block is the 6 small steam holes in the block deck between the inner cylinders.
Used to be the maximum safe overbore on a 235 is .125,and that depends on an absolutely perfect block with no core shift.
Best to toss the block, they are out there to find, just check the Swap Meet here on Stovebolt.
