The reason for the different rear backing plate part numbers is because the hole in the middle is larger for the larger rear wheel bearings to pass through when you install the axles. The GS wheel bearings won't fit through the hole in the backing plates from a standard Skylark. Other than that, I haven't noticed any other differences in the rear backing plates, although I never thought to check the material thickness. The bolt mounting pattern is the same--the gaskets from a regular Skylark are the same and fit with no problem.
I need to get some pics of my car and put them on here for you guys--I recently bought a set of the correct Firestone Bias-Ply tires and put on my '65 GS. It looks a little different from the rear, but not much different from the side since they are blackwall. But man does it ride different though! As with all bias tires, they get flat spots since the car sits around in the garage so much!
