2003 Pontiac grand am 3.1L V6
I have that same car, Mechman has a direct bolt on large case alt that fits, it does 165amps at idle and 270amps at 1800rpm. It is 449 dollars, IMO that would be the best bet, especially since if you upgrade in the future all you will have to do is add a battery in the trunk. So I would do the alt first, I wish I would of went that route when I had the funds.
off-topic
Thats ironic lol I'm buying that DC lvl 4XL 15 from 99GPX that he got from you
It is definatly a task to get a box that fires into the cabin sealed off with it but is doable, if you want I can get you the outside measurements for the box I had in there, I had to take off the taillights to get that extra half inch on eachside but it sqeezed in there with help from a friend, IIRC it was about 3.3-3.4cubes after sub/bracing/port displacment using 2 4" Aeroports tuned to 28hz.
That would be helpful man I never thought of using aero ports for the port and yeah send me those measurements so I can see how you did it.