The paint should have a little more saturation (actually, maybe less saturation) and be darker, and everything should be painted with Plymouth's trademark color except for the wheels. The hubcaps on the wheels should be painted, too.
Another shot of the tractor, and it seems those... thingies on the middle of the sides on the cargo truck should slant downward at the top:
And it should have more specularity than the graphics in-game did. Compare this art below to in-game:
And I'll post this, too, for the hell of it: