You probably have purchased a 57-59 bed. On those the rear sill brackets are about an inch shorter than the 66 brackets that you need. The brackets are easily replaced with the correct brackets. Mar-K has replacement brackets or you can use the ones off the original bed. If the bed is 57-59, you also need to check the fender location, as the earlier bed had the fenders located farther forward on the bedside than the 66. Also on the earlier bed all the bed strip and bed mounting bolts are in different spots than the 66. There are a lot of subtle differences in the beds even though they all appear to be the same.
Good luck on the project.
Mar-Ktech
