New Engine backfires?

I have a 73 AMX Javelin with a 360. Engine was rebuilt about 7 years ago and fired up but then had clutch issues. Its not been started since until now. For the most part it runs good if I ease into the throttle, but if I rev it up right from idle it will backfire and shoot a small flame out the carb(holly 650). All the old gas was drained out of the tank. Is this a carb varnish buildup issue or do I have a intake valve stuck open? Both? I'm guessing the carb needs to be taken apart and cleaned for sure. But will that fix my problem? Is a stuck valve easy to fix? Would it cost a lot to take it in for this? Any advise is greatly appreciated.