Sorry your new paint job was ruined. I had the same thing a while back. It was caused by some flammable plastic tubing catching light, but I dont know what ignited it though.
Yours sounds like a fuel leak problem. Check if theres any wetness around the carb. Check fuel hoses and unions, but a high fuel level in the float chamber is also a common cause of leaks. If its a Holley carb its an easy adjustment, but the original Autolite carb needs to be opened up to reset the level.