Sounds like what was happening with my buddies 66 dart. We adjusted the valves (PO set them too tight) and set the timing and it runs great now. There is also a second adjustment for the timing bracket on the carb. If you pull the dist and look underneith there is a bolt that holds it, that will give you a little more adjustment.
Makes sure that #1 is at top dead center and that the rotor is pointing at #1 onthe cap when you drop it back in.