I didn't run a model, but according to the Eminence white paper, you should be fine with 2.33 internal. In WinISD did you account for internal airspace consumed by the driver (frame+magnet+cone)?
Should be a great cab. Can absorb a full 250 watts without having to worry about excessive infrasonic cone displacement.
You could make it slightly larger to go a bit deeper, at the expense of less power handling down low, but in the real world I don't think it'd make a whit of difference. No one's going to hear a dB or two extra kick at 50-80 when the dance floor's full and half the people are drunk. I'd rather have the Xmax buffer of the box you plan to build. It depends on what amp you're going to use, though. If it's an amp with only 100-150 watts, go a little bigger, just don't loan the cab to someone who uses a larger amp. Basically you can't design a cab w/o envisioning what amp(s) (or BGs, or player styles) it's going to be used with. A cab is part of a total system. But you should at least consider the amp's max output power in your design decisions.