Don't think I had a backing plate either. I just mounted it to the 6 bolts in the firewall that were already there. Be sure to get the adjustment on the rod that fits into your booster adjusted correctly. If your rod is adjustable like mine. You may have to adjust it a few times to get it the way you want it. You'll love the Power Brakes. I am running Power drums and it stops 100% better. Keep us posted.
Good Luck,
Blue71