The body pros on here suggested curved scissors and a dremel with a sand drum for the wheel cutouts and a hobby knife to score the straight lines then just snap 'em off.
I'm no pro but that's the way i do it. I can't live without my curved scissors and dremel.Thats the ticket right there! Curved lexan scissors, and then a sanding drum on a dremel to finish it up. Drill your body post holes, antenna hole, and exhaust stinger hole too before painting. I find fitting the body while it is still clear is by far the easiest way to do it.