Not negative comments but some thoughts:
1. Here is a shot of the real proposed Bradley ambulance:
2. Since you have a "cut down" version of the ambulance, it would probably be like the soft top ambulance version of the HMMWV...
IIRC the soft top version held either two stretchers, one stretcher and two sitting, or 4 sitting in the rear compartment. Again, not a criticism, but that guy in the top stretcher is going to be bouncing off the roof over cross country terrain.
But do not let any of this make you feel bad. You have done a very good job thinking outside the box on this. I only point this out because I had to serve near Medics, so I got to know their vehicles well.