you could have just modifier your backing plate by cutting it to match your axle retainer, then welded a caliper mount onto the axle and used a f150 rotor and 84 cadillac rear caliper and been done for half the price.
My guess is for this kit your going to have to pull the shafts and take the backing plate off, the kit will probably use some type of shim that is the same thickness as the backing plate to keep the same load on the bearing. then a bracket will probably bolt on to the inside of the axle flange.
Honestly.. call the manufacturer, you paid a lot of dough for this kit, they will have technical experts to help you over the phone.