Well, they have the models you can extract from the game...what if you brought that into a program, scaled it up, and then used CAD to make templates from those models?
I'm not sure what a rock truggie is...but I agree on the Hummer/Unimog axles. Would a fiberglass body withstand daily driving and such on the road?
I'd want it to be a car that one could consider street legal, and wouldn't get stopped by the police for "issues"