I haven't read any questions to how stable fuel pressure is during a run.
Did I miss something?

Customer isn't interested in spending more money to pick up a few hundred RPM. The engine makes 740 hp at 6000 RPM which is more then he needs on the street anyway. If he decided to go racing he can switch to a solid roller.

Fair enough but certainly no need to switch to solid roller to run past 6k (or 7k).

I mean... that thing looks like an ancient holley pro-jection system? What old warehouse did you guys unearth it from? :P. Also if it's a true EFI manifold running a carb on it is going to cause fuel distribution issues as they're not designed for it. Would explain your low RPM roughness

It is the Holley MPFI kit for BB Chevy engines. It was recently discontinued so not on their website anymore. The intake manifold was an EFI version of their Strip Dominator. I'm not a fan of this kit but it is what the customer bought and brought to us to use. In the end it made decent power and worked fairly well at WOT but always ran rough at fast idle. We'll see how it works in the car. Not much we can do to fix it since I think the problem is inherent in the intake manifold and/or the fuel rail design. Could be why Holley discontinued the product.

Ah so it runs rough with the EFI on as well? Probably worth a look in the cal to see what the injector timing is and also spark advance...