where are you trying to fit it mschaffer ? In the duramax ? Is it ext cab ? are you trying to keep it stealthy ?
Kinda already said but i think its backwards to use the old or wrong box since they are so cheap (compared to the price of the sub and amp), it will force a bad compromise on the sub you can use, and heck you can make a sweet custom box with 1/2" MDF and fiberglass it up so its rock hard. Then you can get the sweet sub your ears want.
CUSTOM CUSTOM CUSTOM