You’re right, 13” backing plates are hard to come by outside of searching salvage yards, especially the early ones like you need where the brake cable uses two bolts to hold it into the backing plate. Personally, I don’t buy into the rear disk brake swaps. Contrary to popular forum belief, the Eldo/Toronado rear calipers didn’t work worth the hoot on Eldo/Toronado’s where the last place I would use them would be on a large 4x4. Using the GM truck front calipers are better than Eldo’s but they are still a bit small for a large truck plus you also lose the park brake. Nothing will stop a large truck better than good ole 13” rear drums, that is if you don’t mind the extra weight of drums. There are reasons why class 8 semi’s still use drum brakes today and not disks.
If you happen to have the later design backing plates where the cables just push in and held by tangs,
UpTime Parts sells them. I believe these are brand new but could be reman. UpTime is a parts supplier to the fleet world where many fleets are still running 14 bolt FF with 13” rear brakes in delivery vans, etc. The backing plate is known as a “452” in the aftermarket parts business. You'll notice 452 is also part of the brake shoe part number. It is a Bendix identification number.
UpTime also sell a complete brake kit where the shoes and hardware are already mounted on the backing plate. All you need to do is bolt them on and hook up a brake line and park brake cable. Fleets love this brake kit.
Again, these are the later design plates with the round push in type brake cables. They may have the earlier type as well. Give them a call.
FRN452RK7
FRN452LK5
I went through this same ordeal a few months ago when I swapped on 13” brakes on my Suburban. Finally found a 14 FF bolt in the core pile at a local salvage yard that I was able to pirate them from. Luckily, we don’t use salt here so our junk is pretty rust free.
If your truck is an original '78 axle with 13's this is the backing plate you need where the park brake cable bolts in place. I don't have a picture of the round push in type brake cable backing plate. BTW, you cannot easily convert the bolt on brake cable to the round type easily without changing the entire brake cable assembly from the park brake foot lever all the way to the backing plates.
