1998 K1500. Stock wheel size on the door is 16x7 6.5J. I get the 16x7 part but what does the 6.5J mean? The rims I'm looking to get are 16x8 Will those fit the tire which is the stock 265x75R16? Im a ruh-tard when it comes to all these numbers

 

The 6.5 should be the back spacing. You shouldnt have any issues with a 16x8. Just make sure that whatever you put on will not rub.