I swear is everyone in this thread mathematically retarded? 6" Top Depth, 9" Bottom Depth, 15" Height, 55" Width. Subtracting for the internal dimensions gives you:
4.5" Top Depth, 7.5" Bottom Depth, 13.5" Height, 53.5" Width.
(4.5+7.5)/2 * 13.5 * 53.5 = 4333.5 cubic inches
4333.5/1728 = 2.507 cubic feet.
Thus, those dimensions you gave give you approx. 2.5 cubes before any sort of displacement.
If I were you, I'd run 6 8" Ascendant Audio Assassins in a sealed box of the dimensions you gave that are available. That'll be louder than anything mentioned in this thread.
Or, 3 10" Arsenals from Ascendant Audio sealed would be nice. From my experience (I own a 2000 Reg Cab Silverado and have experimented with **** near every combination for a subs/amps/boxes you could imagine) Cone area and a sealed box produces the best results for this vehicle.
If you want more help, pm me and we can chat on AIM or something.
edit: Hell, for a couple dollars, I'll design you a pretty badass box for your regular cab