Try to keep the wheels as close to the perimeter. If the game involves a ramp of some kind the wheels will come off the ground when the front or back tube hits the ramp. You might try running the front/back tube at a 45 deg angle.
Here is a picture of what I am talking about
:
https://picasaweb.google.com/1177698...07353715384594
If you can bend flanges into your G10 belly plate it would strengthen the pan considerably and give you a flange to rivet the plate into the side tubes.