On pg.15 of Howard Townes "TWO GREAT TRUCKS" he gives a pretty good explanation of the differences, on page 16 has a picture of the car vs commercial fenders.
It mostly states "they differ @ rear where they attach to the cab. The car fender curves out to follow the bulging body whereas the P/U is straighter so it fits the flatter cab sides"
You can get that book off the Early V8 web site. Scott