Measure the old rotors and see if the new ones are the same diameter.
We have 1500, 2500, and 3500 to choose from. The 2500 means nothing for brake parts as GM used systems from 3500 on 2500 etc. The difference is weight on the door jamb 8600 being a "heavy" 3/4 ton or singled out 1 ton. 6 or 8 lug also makes a difference. 1500 and 3500 are just as mixed up. Then you get to the rear drum size and that can make a difference on the front brakes equipped...
What store did you get them from? You may need to look it up differentially or go to a different store. Dealer prices for rotors are not too bad and you get the right parts.