11" Backing plates rubbing

Disc brake A-bodies were already overbraked in the rear with the 10" rear drums they came with (real easy to lock up the rears, fixable with 13/16"-bore rear wheel cylinders this way). 11" drums are going to massively aggravate that problem. If you're bound and determined to run the 11" drums, put the even smaller 3/4"-bore wheel cylinders in them. But it's still pointless extra weight.