Newbie needs help with /6 tuning

Although very nearly every Holley carburetor I ever had my hands on (a bunch shall we say) ran rich (slightly to a lot) on the street with the factory jetting (about 2 to 4 jet sizes too large being the norm), I don't think that sounds like your problem. Even with the factory jetting (again rich), you should be able to have some control over the idle by adjusting the screws. Also, the odds are, if the float is bad, or the float adjustment is off enough to cause what your saying, it's going to be puking fuel out the vent.
I'd be looking at a blown power valve. Holleys (especially the 4150-4160 four barrels and their two barrel derivatives) are known for this, to the point of advertising their newer carburetors as having blow proof valves, and having a kit available for modifying older carburetors to prevent the problem. If the power valve goes (if your motor has too low a vaccuum at idle for the # valve you're using, it will act the same way), there's no way in hell you'll keep it from running pig rich at idle, short of dropping the main jet size so far you get lean misfire when the accelerator pump shot runs out every time you accelerate.
Everybody is a "Holley expert" and it sure sounds one of those experts has screwed with yours, but no worries, odds are if you do your homework, work systematically, and buy a few parts, you'll get yours back into shape.
I bought a used one once REAL CHEAP (5 Bucks cheap), because it had the power valves (there are two in a 4150) replaced with plugs, and the jet sizes increased to compensate. It ran so rich, your eyes would tear up standing next to it. Because it idled so rich, the plugs would foul so bad the motor would hardly run. The guy tried all kinds of plugs and couldn't keep any of them in it without fouling. He couldn't figure out the problem, so of course with the car running this way, the carburetor was "defective". I certainly wasn't going to tell him it was an OWNER malfunction.
Turned out to be one the best running Holleys I ever owned once I got it squared away.
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