I had an overcharging and starting problem with my 71 Challenger. Once in a while it would not start. A few times I ended up "pop starting" it. The voltage was low at idle and way over when revving. I'd blow the caps off the battery. I couldn't find the problem and had 3 different garages hunt for the problem. No one found anything. One day I was under the dash and just happened to notice the connector plug for the steering column to the dash harness had a black streak in it. It was the heavy red wire. I clipped the wire on both sides of the connector and bypassed it with a separate connection. It was fixed. No more problems with it not starting and overcharging.

So look for a bad connection. Especially in your hot wire from your starter to the relay.