Alt advice

Switch over to a modern alternator like on a 90 dakota or similar.

Older Mopar alternators do not generate enough wattage at idle and you cannot engineer that out of the design with a 100 amp mod. It helps a little, but not much. I have a very nice tuffstuff 100 amp mopar alternator and the voltage still drops badly at 600 rpm idle. Voltage is 14.5 at 1500 rpms and it goes back to sucking at idle.

You didnt mention if you have upgraded your wiring but running a 6 gauge from your alternator + to your starter relay post + and deleting the runs to the amp gauge is a much needed start. Mopar wiring system is sadly anemic from one end to the other.