one thing to check, i think that has HEI distributor with vacuum advance? if it does what happens sometimes is there is 2 wires in dist that go from the pickup coil to the module. every time the vacuum advance moves it wiggles those wires. then they start to break. and when you use throttle and the advance moves they loose contact and cause a bad stumble. i would unhook the vacuum advance line and plug it and try driving just to see. or you can pull cap and tug on the wires. they always crack right where they go into pickup coil. but keep in mind if that is problem they will a lot of time break completly when you touch them, then its dead. just one idea to check.
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