Electrical charging problems

A 42 amp alternator is going to have some seriously low, low output at idle, especially at 600 rpm.

I wasn't even really impressed with the 78A square back when I had one, but it would probably be adequate.

The first thing I would do is look at the voltage at the back of the alternator vs at the back of the battery. There shouldn't be more than about 1/2 volt difference with the engine running. If it is, the wiring is suspect.

I would replace the wires in your charging circuit with some 10 GA wire. The factory ones are very small.

If you have a NON-A/C small block, it's super easy to convert to a Delco 12SI which will charge extremely well at idle.