235's had bores of 3-9/16, and 261's stock bore was 3-3/4. I had a '54 with a 235 but never paid attention to serial or casting numbers, I never knew about captain bars 'til I got here,maybe thats why the strip inspectors always made me run in a sportsman class after I tried to claim 235, when I put my 261 in it.
I know the 261s had their year cast in the inside of the crankcase, In 1959 they started adding the full flow oil filter provision, so this was an easy id to id the long shaft water pump 235/261s "55 thru '62. I just dont trust the casting/serial number stuff.
