Ok...so, recently my son's '73 Roadrunner with a 400 has been having some issues starting cold. We're in AZ, so cold is not really cold. Only when the car has sat long enough to cool down it will not start or fire. Thinking it is a fuel delivery issue with gas disappearing from the bowls (Holley 600) pumping the gas results in a raw fuel backfire through the exhaust when it does light off. I haven't pulled the air cleaner to see if fuel was squirting or not, but the 20 foot flame it threw tonight out the back and the deafening boom that scared small children in China tells me it's getting plenty of fuel. Once it starts it runs great and starts every time as long as it doesn't cool down. Any suggestions what to look for? I'm trying not to jump in blind...

Choke? Most BB Mopars I've owned had a tendency to be "cold natured" and were very dependent on the choke working right.

My second guess would be the carburator base gasket causing a vacuum leak and messing up the air flow through the idle circuits. I've had that happen too. But then you say it runs fine when warmed up.