High end miss - need help

[Botietruck]

My son has a 71 truck with a 350 engine/th400 trans. A couple of weeks ago, the truck acquired a high end miss. Basically, either at idle or on the interstate, as long as he was barely on the gas the engine would run smoothly (even if he goose it while in park. Whenever he would step on the gas, either at low or high speeds, the truck started vibrating. It progressed into a high end miss eventually. Now it's at the point to where at idle it runs moderately smooth and if he gooses it while it's in park it will pop and backfire. If he drives it he can barely get it up to speed. Any ideas from anybody? Already replaced spark plugs. He had intended on installing a Holley so he went ahead and took advantage of the situation. No change. Vacuum advance? Timing issue? Valves? Any thoughts or help would be greatly appreciated.