I have since cured my problem...
I suspect it was a combination of things. I replaced the original coil with a new one, added a heat shield, reset the timing and then fine-tuned the Holley with a vacuum gauge, and my starter had slowly been dying, getting slower and slower when cranking. I put a new one on and it spins over like crazy and fires right off.
I also had a charging issue after the motor mount broke and the alternator's positive post hit the inner fender. The regulator wouldn't allow more than 12 volts, so I replaced both of them.
Thanks to all who offered advice.