charging issues

If I can jump this thread, 74 Dart, bat. in trunk, two post kill switch, stock elec. dist, older msd-6 box, elec. fan and elec fuel pump. I replaced the volt reg. when I first got the car because of excessive voltage. The car has sat for over a year since then. I have put in a new agm battery, checked voltage 12.3 volts at the bat lug on the alt. Had the alt tested, only making 10 volts. Put in a reman alt. still only 12.3 volts and no change when I raise the engine rpm. I put the battery on a slow charge and it is up to 13.5 volts. The P.O. has bypased the amp gauge by splicing the wires together and run a jumper wire to the starter relay. No fusible links in sight. The wires through the bulkhead look sketchy. I've done the madelectric conversion on my R/T, but it doesn't have a roll cage or valve cover 1 inch from the bulkhead connector. I'm going to have to get a good night's sleep and find some patience for this job. Anything else that might cause this? I don't want to be the guy whose car won't start in the staging lanes, again.