Slant 6 No Spark

This sounds about right. as you said it only pegged when driving. at idle it was at moderate charge.
OK. Getting closer.

Since its turning over, the battery's not dead.
But at some point check the battery condition and charge level.
If its a traditional wet cell battery, check the acid level. Should cover the lead plates. If not add distilled water.
If its a sealed battery obviously can't do that.