The guts are swappable with later parts. Even considering the straight cut vs. helical gears I believe they both fit just fine. My problem was that the cast iron body was damaged on mine. The mounting holes and alignment is different on the 55-57. I bought the newer style pump and tried to make it work but it just would not line-up. This has been researched by others also on the Pontiac bulletin boards, but I had to see for myself because of the difficulty in finding the correct body.
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