Alternator wiring

If you are using the "normal" vintage Mopar alternator, it is well documented that they don't charge much or at all at low RPM. If it works "as normal" off idle and cruising, the setup as you have it should be considered normal

You might read this post to see how the overall system voltage drop is doing

http://www.forabodiesonly.com/mopar/showthread.php?t=141911