HEI Swap with backfiring through the carb

[thepunisher]

My brother and I started an HEI swap on my 1972 C10 on Saturday. We ran new 10 gauge wire to a known working distributor. To set the engine at TDC we pulled the valve cover and turned the motor over till both rocker arms on the #1 cylinder were loose and the mark on the harmonic balancer lined up with the timing marks. We then installed the distributor so the rotor lined up with the #1 cylinder and was pointed as close as possible to that point in the cap. We then installed the wires in the proper firing order. When we tried to start the truck we got backfiring through the carb. To try and get this to stop we both advanced and retarded the timing by rotating the distributor. As I understand it I should be advancing the timing to get this to stop. My questions are this;

Am I right on our method for TDC?
How much do I have to advance the timing do get this to stop?

(Our first attempt at stabbing the distributor was 180 degrees out and we got back firing through the exhaust)