I just went through this after replacing my cam.. put my distributor back in "where it was" and first crank, backfire and flames through the carb which lasted for a few seconds while I got my extinguisher out.

Anyways in my case the timing was very retarded. Turned the distributor clockwise to advance, got it started. When I got the light on it, it was still way far retarded. After that I got it to 20 btdc, the idle sped up a lot allowing me to turn down the set screw, and all is well.

Has your engine ever ran right or did this just start? How does it run once started? Crossed wires would be pretty obvious when its running.

If I were you I'd advance the timing more.