I have gone through a very similar situation with my new 505" 440, 10.2 compression, XS290 solid cam, Stealth heads and basically original 6bbl setup.

A couple of years ago I put the Promax metering blocks/plates in all 3 carbs on the original motor in anticipation of my new motor being done. I wanted to be sure they ran ok before putting them on my new motor and if I had trouble didn't want to troubleshoot any unknowns. The original engine is/was pretty tired and the ran same with the promax stuff as it did with bone stock carbs. I've had 'em apart, shot carb cleaner through all the ports, new gaskets, etc.

The new motor went in and was fouling plugs, even the hottest plugs I could find. It wasn't performing any better than the stock motor, and probably worse. No good, so I asked for some help and a racer not too far away dug into the carbs. The culprit- the center carb with the Promax metering block dumping fuel. What happened in the last couple of summers of driving? Who knows... He pulled the Promax block out and slapped the stock one back in, adjusted the outboards and now its running like it dyno'd - 590hp & 650tq.

I just drove it home last night.

Hope you find your solution soon.