327 idles great, backfires with throttle

[1986c10]

Hi, I picked up a mint condition 1979 C10 project that has traded hands alot and was never completed. The engine was rebuilt in 1998 and never started, its a .040" over 327, small cam if not stock, intake, headers, holley 600 double pumper etc. I bought it not knowing if the engine was junk or not. I changed the oil, rebuilt the carb because the floats were stuck, distributor was somehow bad, replaced that. Fired the truck up and broke it in for about 15-20 minutes. I set the timing at 14*, adjusted the mixture screws on all 4 corners of the carb and it now starts and idles great. Problem is, when you smack the throttle it backfires or just hesitates, so bad that i couldnt drive it up the slope in my driveway without it stalling. I sprayed carb cleaner on everything, found no vac leaks, i even blocked off the brake booster and pcv with no change. Vacuum from memory was kind of low, like 12" i think. I bumped up the carb squirters from 21 and 28 to 28 and 31 with no change, ive advanced the distributor more with no change. I'm kind of getting frustrated with the thing. Anyone have any thoughts? I'm debating on slapping a friends carb on it just for a sanity check.