Doesn't sound good. Better read up on this topic at Mad Electric. It can point you in the right direction.

http://www.madelectrical.com/electrical-tech.shtml

Mopar alternators are terrible at producing power at low/ idle rpm, and not wiring them correctly to a rear mounted battery can make things worse.

I just finished upgrading my charging system with a CS144 alternator. It pretty much bolts right on and puts out up to 120 amps at idle, and maintains 14 volts at 850 rpm with both electric fans, my electric fuel pump and MSD running. Over 1000 rpm it holds 14.55 volts and is capable of supplying over 170 amps. I'll post a picture later.

1970 Plymouth 'Cuda #'s 440-6(block in storage)currently 493" 6 pack, Shaker, 5 speed Passon, 4.10's
1968 Plymouth Barracuda Convertible 408 Magnum EFI with 4 speed automatic overdrive, 3800 stall lock-up converter and 4.30's (closest thing to an automatic 5 speed going)