What would make a 350 chev carby backfire at 3000rpm but only while in gear?

The most common reason for backfire in an engine is the timing is not set correctly. The bad timing could also be caused by the timing chain beingworn out or a camshaft that is worn out. Get a timing light and check to see what the timing is set to. It should be around 8 to 12 degrees before top dead center at idle(800-1100 RPM) with the distributor's vacuum advance disconnected for most Chevy small blocks. There should be a sticker in the engine compartment that gives the idle speed and timing spec. for that model. If there is not sticker, the spec. above should correct your problem if the parts mentioned are in good condition (not worn out).