Need help with missfire under load on 340

Hey everyone ive been having this issue for awhile but today it seems worse, 340 with 4 speed. It will rev up fine sitting still but under load at above around 1500-2000 rpm you can hear popping coming from the exhaust and feel like its surging, swapped the chrome box for a orange box i had same issue. Timing is 16 btc at idle, all in at 34-36 with vac advance disconnected, battery voltage is good off at 12.76 volts. alt putting out 13.6-14 volts. Plugs, cap and rotor are only about a year and a half old. I can especially tell when i go to take off it wants to buck and fall on its face. Cruising at around 2000 fells normal until i give it more throttle then it pops and crackles from the exhaust. Any thoughts? I appreciate it.