Tell us about the engine. Mechanical tappet much like the 375hp or wild, or stock? Is the idle normal? For the build… smooth if stockfish cam? Normal idle if hotrod cam (i.e. idles the way it usually does with that cam). Is this a setup that’s been on for a long time and the problem just arose, or is this a completely new installation? If a completely new installation, is it all numbers correct, metering blocks, jets, carb bodies?
Shooting off the cuff, and if the idle rolls and is fat, It sure sounds like a power valve issue. If not, how about fuel pump? Mechanical running normal pressure, or over fueling with a high pressure electric? Food for thought. I’ve run a totally stock setup on a mild 390 with great results. Neat setup, one of my favorite.