Electrical charging problems

I do have factory pulleys with new belts (tension on pulley is acceptable), it has a 42 amp alternator from autozone (duralast), engine idles at 900 rpm at park/ neutral and 600 rpm in gear, battery is 2 years old, and all I have for accessories are my general lighting (headlights, driving lights, tail lights, blinkers, cab clearance lights, and an after market radio) even before installed the radio my jalopy had this problem. I downloaded a 75 dodge truck service manual but it does not read anything about my situation.