1968 Ford Mustang Carb. Problem?

Engine Performance problem
1968 Ford Mustang V8 Two Wheel Drive Automatic 105k miles

Hi, my name is Josh, I am 17 and need some help with my car. It's actually a 68 fairlane 500, but it has the same engine as a mustang. My question is why is my engine backfiring inconsistantly? It idles fine, but when I goin gear to about 2k RPMs sometimes she'll pop, and backfire out either the carb, exhaust or both. Or when I gunn it it will pop, and when i'm going down the highway it will pop decellerating. Sometimes even sounding like a missing issue at the higher RPMs. This has a '68 302 bored.40 over, HO cam (firing order 351 w), msd blaster coil, points and condencer(gap at 020) an edlbrock 289 intake man. With 2" spacer, and a holley 4160 600cfm with vacum secondaries. I also have long tube headers as well. I really appreciate your time reading this and hope that you can certainly help me out. Thanks again, --Josh