340 tunnel ram issue

i have a 1973 dart with a mild built 340 and 4 speed. the 340 has a weiand tunnel ram setup with dual holly 4brrl's. i don't know all specs on motor because i did not build this car and previous owner got it in on a trade and i got it off him, you know the story. anyways my issue is that the car ran great when i got it but lately now i cant get it to do much at all. the car will start and run but will seem like it is missing and when you go to give the car gas it will pop and stumble and sometimes backfire and sometimes backfire from the front carb on the tunnel ram. i have checked the plugs and changed them already and also tried to play with the timing. another weird thing about my issue is it seems like when i mess with it and i think it's about right ill test in neautral and rev it no real issues so i go to drive the car and as soon as i let clutch out fully and goto give it gas it will do same as before back to backfiring, quiting, missing. just seems like anything rolling over 1k rpm has issue. i am lost and getting annoyed with it haha. i need some help please...