The shoe backing can easily be reused with new linings riveted on. Just about any large truck shop should be able to do that, but I know there are a number of places you can ship them off to that can quickly do this as well depending on your situation. Same thing with the brake band on your transmission. I have also not successfully sourced the return springs for the rear either, so be gentle to them.
I typically strip every brake part I can from any of these trucks I find in the junkyards. Its not often, but its the best way to get drums, shoes, springs, adjusters, etc to have spares on hand and to help out others. May be worth calling around, but fair warning 8 times out of 10, the front desk has no idea they have 50/60/80 series trucks in the yard.
