Since the 261 is the truck motor brother of the 235, I assume the same holds for them as far as crank design. Unsure if its the same crank as 235 or if the 261 had a longer stroke. The heads will interchange, except for the steam hole issue. The differences are that the 261 used a full-flow filter, and had a later lifespan, like 55 or 56 to 63. I think the 261s all had 12 volt electrics and 2nd series oil/water pumps. But never having worked on a 261, this is mostly parroting others here. But I would assume that all 261 cranks/rods should interchange in a 261.
Back to original poster RonB, keep asking questions here, as there are about 20 different modifications you can do to a 235 (or 261) before you assemble it to make it run better and last longer.
good luck
chip
