alternator going out?

i had a slant six in my duster and now it has a 360 in it, i upgraded to electronic ingnition im using the altenator off of the slant six, the build plate says it is a heavy duty and has the big box windows on the back, i have also upgraded the head lights to modern bulbs inside fixtures and have put in a newer radio, and the car also came with ac, i dont know if any of this affects it

the problem i am having is that at night i noticed when the headlights are on they are dim at an idle then when i push the gas they get brighter and during the day the gage is all over the place at different times