66 Dart GT HT Whatsitgonnabe?
Is it electronic ignition - have you checked the reluctor gap with a brass feeler gauge - not steel. Or still points and again checked the points gap? Idle circuit in the carb? Plug wires in the proper firing order? Have you determined true top dead center and then checked where the timing mark is on the balancer? it might be too retarded. Did you replace the timing chain? If not could it have jumped a tooth or 3. It sounds like a timing problem. Just throwing out possibilities here.