Aren't those specs for the new T400-4? I thought it was something like 25x4 at 4 ohms and that they're stable at 1 ohm. I could be wrong though. Could you post the birth sheet?
Edit:spelling
He has 2 Crescendos and you forgot to include efficiency. With the amp draw and about an additional 50amps for the car, you should get a dual alt kit (like Kranny said)
I would do a sealed box. How many cubic feet could the box be, how much power do u wanna run, and what is the maximum mounting depth that the sub can be?