all the ones i have seen ARE marked front and rear. but i have never seen any kind of spacer. the differance is the offset.
gm stopped having diff (front vs rear) when they went to the 4th gen with the front wheel drive rims. but if you look at a 4th gen you can easily see the diff. the rears sit further inward.
this is just a random pic i found doin a google search, but you can see how the front sits even with the fender and the rear is in more. its the same on a 3rg gen but with the iron and gta wheels they compensated for it.
her's an 85
and here you can see on an 85 with 16 inch wheels the rear sit out furthur
