Probably the wrecking yard is your best bet on backing plates. One of the bad things about owning and repairing the big trucks (8 lug 2500/3500s) is there weren't as many of them made as the half tons, so parts are more scarce to begin with. Then factor in that these were, and still are, work trucks and so got the snot run out of them on a regular basis. So a lot of the parts we now need are worn out, or close to it, and the supply of replacement parts has been depleted too....
My one ton is in very good shape, being a South Texas truck all its life and having low mileage. But I do see it as an investment and plan to upgrade a few things, and be sure it's properly maintained. We're having a rainy streak for the next several days, so an oil change is in the plans once it's dry again (I have to work on my trucks in the driveway).