Difference in fuel pumps?

Is the return spring in good shape and correctly installed. The pump arm has to remain in contact with the eccentric,lol. The spring sets the pump pressure.
Any cover is ok. As is any pump with a similar arm.

I suppose you have proved the hard lines are clear and all the neoprene jumpers are intact and properly clamped and that the sender is not compromised, that the tank is vented and that there is actually fuel in the tank,lol.

If you fill the float bowl manually, the engine should run for a minute or two; enough time to do a pump volume test. Which should free-flow output 1 qt, in 1 minute or less, at 500rpm.And it should pull a 10 inch vacuum, and make 5 to 7 psi; on factory SBMs.