Why does my '77 GM pickup miss when running?

I have a 1977 GMC pickup that starts and idles just fine. All seems fine till i get going in 2nd gear. It's a standard transmission and the problems seem to start when I get my rpms up in 2nd gear. I get enough rpms to switch to 3rd but when in 3rd the problems seem more apparent as the issue starts at lower rpms. The engine misses in 1st as well but I really have to get those rpms up there. I have already replaced the fuel pump, fuel filter, accelerator pump, spark plugs, spark plug wires, distributor cap and rotor. It's an old farm bucket with Low, 1st, 2nd and 3rd gear. 1/2 ton with a 350. It has a rochester Quadrajet carb. I don't know if its so much of a miss as much as it bogs down but it sure kicks when this happens. I don't know if there is any more information i could put on here but if you need to know more please ask. I want to make a project of this truck.