What kind of motor does your grand prix have? I have an 81 catalina with a 455, the stock alts on those cars are junk, but you can get a 180 amp alternator for a police "high output electrical system" option caprice for about 60$ at autozone. Depending on your motor you can have room for dual alts, or not.
This box is 8 cubes, all the equipment is in my sig. You should be able to get something loud in that trunk, i assume its close to the size of mine.
Did you know that if you have a true pontiac GP (not a chevrolet made one from the 80s) the trunk is considerably bigger? My trunk opening on my catalina is 6" longer and 3" wider than a caprice. (chevrolets counterpart) Doesnt seem like much but it adds about 3 more cubes to the trunk.