alternator causing bad idle??

my car has been sitting for about 3 weeks so took it out for gas tonight and the lights are very dim and the idle is ruff like it wants to die when the lights are on. the ampmeter is showing between the middle and far left and barely rises when its reved up. i noticed the last time i drove it it was raining and with all the wipers and lights on it would die if i let it idle. runs fine with all accesories off. i replaced the volt reg last year also its a 73 318 factory electronic ignition all parts replaced in last year. could my alternators low charge be causing the running issues? and if i replace it do i need the 50 or 60 amp alt. all wiring is great thanks.