Just for clarification, I'm running triple webers, not EFI. Although you can run carbs on a p79/p90 you *might* run into sealing issues with the injector notches. most likely not though since a ton of members here are running SU's on their p-series.
As for the mechanical fuel pump I can't help you there, my car came with an electric pump, and I prefer that to my friends mechanical one. Less clutter in the engine bay, and a quieter ride. I would put an electric one on if I were you, but that's just me. If you are on a budget and got the tools though do what blue72 did. My fuel set up cost me like $230. That's the pump, filter and FPR.
did you buy a long block or did it come with intake/exhaust manifolds on it?