Thanks to all the bros on the forum for your advice. Been working on my truck the last couple weeks and today I got it done. I put the 4412C Holley carb on with a modified "Transdapt" adapter on it, new vacuum advance, set the timing, and my buddy helped me set the valve lash to the recommended gaps. It was backfiring, we think, because the valves were a little too tight.
I drove it 3 times today, starts, idles, drives perfect, and NO more backfires thru the carb. I'm stoked!
