OldIHC

Has anyone had problems with sufficient fuel delivery and found solutions?
The after market pumps are lacking. I believe we've ruled out all other causes and singled it out to the pump.

Did you actually "flow test" the pump, to see how much it delivers in a set time with a set rpm?
I've had pin holes in the fuel line not leak gas, but suck air in, cause a problem.
The cam lobe can also wear and reduce pumping action.
Other causes can be restricted fuel line or standpipe in the tank (if used).
Fuel lobe on cam can be checked with a dial indictor while the engine is turned slowly.
You say "aftermarket pumps are lacking" which implies you have tried more than one. If you replace the same part Mutiple times for the same problem without curing it, chances are the part you are replacing is not the problem.
What are the symptoms?