Camo, Just go for it. If needed, you can always CAREFULLY move the driver's seat back. Granted you may not want to do that, but its an option.
The other thing that you could do is to take all the XYZ measurements for the driver's "compartment" in the HMMWV, scratch up a mock-up real quick, say out of styrofoam, and use this during the construction of the figure to make sure that it fits.
If you find that its a tight fit, don't worry about it, so do all the real drivers in Iraq/ Afghanistan!