If Greg_H54 is right about the sort of replacement crate motor, using the small lines and early water pump makes perfect sense. You get a 261 and swap all of the accessories, including the oil filter if the original motor had one.
And just as an fyi, Pontiac of Canada used 261 motors as their base motors in those years and they used small lines. I have 2 59 Canadian Pontiac 261 blocks, one in my 57 chevy. Both use the smaller lines.
