Here's a few ideas that may help:
Try to find the same brand and number as your old pump -just to compare if nothing else.
Call yet another parts store (an independent preferably), give them the number off your old pump and ask them to cross the number over using an actual paper catalog-not a computer.
Check several brands of pumps, I've noticed pump arm styles vary with each manufacturer.

I just took a walk around the machine shop and happened to find a 460 that was already apart and a fuel pump looking almost identical to your new pump was in the pile of parts. I'm assuming the pump belongs to that engine. I could barely make out the numbers but I believe they are #4897. Perhaps you've ended up with a 460 pump.

good luck