[KB66]

I would use the ProVehicles outline for the Hummer.
Blow up the photograph to match the vehicle outline. You may need to go outside the original aspect of the photograph to make it fit proeprly. If that's what it takes, you don't have much choice. Typically if I had to adjust scale, I'd explain to the customer and make sure he'll be ok with it. Don't forget you can go outside the design of the hummer a bit, try to make the ferrings line up with the wheel wells. You may have to do some cutting and pasting to have the engine and sides in the right place.

This is a job that requires high end design skills.