68 Valiant dies and runs lumpy, then suddenly runs fine.

Sounds to me like it's time for a new throttle shaft. Look at the outside of the carb near the base. If it looks like fuel has been leaking down the sides at the ends of the throttle shaft, it is worn out and you will need a new shaft and bushings for the carb base. To tell for sure, start the car and spray the base of the carb with starting fluid or use an UNLIT propane torch that is cracked open and aim it toward the base of the carb. If the engine smooths out, you have a leak around the throttle shaft. You'd be amazed how much air an engine can suck around a loose throttle shaft.