I thought I had my electric troubles solved on my 63 Polara 500 (383), but I guess not.
The symptom is getting intermittent spark. Battery is strong and I have a jump box on it. It cranks strong always. What I noticed was that sometimes I get the oil pressure light on and gas gauge moves. But sometimes not. If the oil light is on, I'll get spark and it will fire. If the oil light is not on, it will crank but will not fire.
At first I though it was a bad switch. It seemed to me that jiggling the key when I cranked made some difference.