You can’t use those 2.5” backing plates, or any other backing plates from a large bolt pattern axle. They MUST be from a small bolt setup. The small bolt axle and corresponding brake setup has a unique offset that is only available in a small bolt axle. You need to either find a pair from an A body 8 3/4” axle, or you can use the 7 1/4” small bolt backing plates from a 10” brake system, but you would need to drill the 5th mounting hole, as 7 1/4” backing plates and axle mounting plates only have 4 studs, the 8 3/4” has 5 studs.