Can't find misfire

Recently out of nowhere, I have a really bad misfire. It was driving fine, I stopped and shut it off, came back and it as misfiring really badly. It does it at idle, cruising, and light acceleration, but if you put your foot in it, it clears up and runs great until you let off. I've replaced the distributor because the advance didn't work on the old one, set the timing, did an HEI conversion, moved the fuel filter to the other side of the engine, adjusted the valves to spec (while idling), and tuned the carburetor as best I could. Now, to me it sounds like it's ignition related, but I'm not sure. The carburetor was rebuilt a few months ago by a friend of mine, but he really didn't know what he was doing. He only did it because it was stuck in a parking lot because it filled it up charcoal from the canister. Now also, it's not always doing this. Most of the time it'll misfire like it's not running on all six, but sometimes it'll run just fine. But turn on the air and that extra load makes it worse. Any ideas would be greatly appreciated before I start ripping into something else. Oh, and if it means anything, I have the Holley 1945 carburetor, idle is set around 800rpm, advance is currently sitting at about 10 degrees (doesn't change anything but idle speed and quality, misfire still exists), stock distributor, valves around 10 and 12 thou for intake and exhaust respectively, I don't know what compression or manifold vacuum is at because I don't have the tools to check at the moment, and the carburetor was tuned by turning the screw back and forth until it ran its best. If I can, I'll re-tune tomorrow with a vacuum gauge and check compression.