As far as the space goes for the 15s, I'm saying that you won't have enough room to be able to give them enough port area to make them happy and tune that low. I owned a 2000 Grand Am and I know there just isn't quite enough room.
As far as you experiencing unloading issues, was that when you played under your tuning or not? Just interested cuz I'm working on my 2 XL 15 box and can't decide how much box volume to give them, I'm stuck between 6 and 6.5 cubes.
And, Yes, Sanitarium has a 6th order and so does Lunchbox.inc, and I'm sure they could give you some tips on how to set one up properly. They are just a little tricky to work with.