I've not tried in-game at all, but having a quick look here's a few pointers:
- you'll want to associated parts of the shape with the name of the object in GP4, as the named objects in GP4 have specific properties, the brakeglows and rear lights come on at different stages, the nose, left and right front wing and fins come off in a crash and replaced at the pitstop etc, same with the rear wing (upper and lower, left and right fins), the mirrors (if mapped correctly) are for the mirror reflections, and visor are for transparent/windowed parts. Anything you don't need (like barge boards and z_cockpit_insert for example) can be just one polygon hidden somewhere inside the car.
- you'll also need to add an alpha layer to the texture, the alpha layer manages the strength of the reflection in-game with white being super reflective and black moving towards matte, so you'll want to start with a fairly dark grey and tweak as necessary.
Anyway, hope those little things help, feel free to ask for any more tips.