'72 402 backfire through carb - at wits end..

[LujMK]

Hi...many years ago, as a gift, I bought my father a 72 GMC with a rebuilt 402 and carter (now edelbrock) 1407 carb. He's been tooling around in it over the years and recently it made a popping noise (per him) and he parked it. I returned from overseas and together we noticed low compression and oil - bad head gasket. We changed both head gaskets (heads were nice!), and after noticing fuel leaking from the carb linkage (manual choke area), father rebuilt the carb. We put everything back on, used the faqs on this site to get TDC, installed the HEI carb accordingly, put the fluids in and attempted to start the truck. What we got was a backfire/flame through the carb a couple times - it would try to start as we'd advance/retard the dizzy but still just skipped and burped out the carb. We're both at a loss on this one. New plugs, new wires, valves (comp cams rollers) were spaced (well - as rollers are) according to the faqs here and we both swear we had the right TDC .. but .. any thoughts? I've got to deploy again in 10 days and hate to have the old truck sitting for another 12 months.

Cheers.