Can anyone tell me the difference between the two water pumps listed in the moss catalog - 434-540 and 434-545. A friend of mine is restoring his car, and put in a 1275 motor. I'm not sure if it's pre-12CD/H1746 or not, and he doesn't know which pump to use. I wondered if the impeller and/or snail is different depending on the block. Phil Bates