Not sure if you purchased a kit from us or not, but no matter.
Carter used pump number 64-328s in the 4242s carburetor.
This pump is the same diameter of pumps used in earlier AFB carbs. This pump differs from earlier Pontiac pumps in that it does not have the internal check valve used in some of the earlier pumps, but still the same diameter.
One of the "tricks" employed by carburetor tuners on both Rochester and Carter carburetors is to simply bore the pump cylinder out to accomodate a larger pump. The larger cylinder thus has a larger volume which will deliver a larger volume of fuel by the accelerator pump.
So, it may possibly be that you have a carburetor that has previously been modified, in which case the pump in any kit listed for a 4242s will be too small. If the pumps you are getting in the kits are physically smaller than the one removed, this is probably the case.
However, if the pumps have a leather cup: the manufacturers of accelerator pumps place a cardboard cylinder tightly around the leather to protect the leather. Leather cup washers SHOULD be SLIGHTLY expanded before installation by running a blunt instrument between the leather and the thimble to GENTLY expand the leather. Once expanded, the garter spring will keep it expanded.
Jon.
__________________
"Good carburetion is fuelish hot air". 
"The most expensive carburetor is the wrong one given to you by your neighbor".
If you truly believe that "one size fits all" try walking a mile in your spouse's shoes!
Owner of The Carburetor Shop, LLC (of Missouri).
Current caretaker of the remains of Stromberg Caburetor, and custodian of the existing Carter and Kingston carburetor drawings.
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