Finally got the bearing replaced, my newly rebuilt backing plates with all-new hardware installed, and the truck's back on the road again, just before it started raining. I drove it around town long enough for the heater to warm up my toes so I could feel them again :) Turns out the machine shop guy was able to salvage the retainer plate by merely flattening it; however, that did force the mounting holes apart and I had to use a Dremel grinder to elongate the holes till they fit the mounting studs. The bearing apparently caused the retainer plate to stretch.
Unfortunately, I can tell the other bearing is beginning to go out, so I'll have to replace it in a couple months after the weather warms up before this happens again. But this time, I won't have to also replace the backing plates. My old ones were so rusted-out there were holes through the left plate under the brake cylinder and I'd had to use large washers under the shoe retaining pins to keep the pins from pulling through the backing plate.
I still don't know whether a 7.5" will swap directly in place of a 6.7", though; apparently, nobody else has done it. I have a suspicion these 9" backing plates fit either the 6.7" or 7.5" as they were originally advertised on epay for a 7.5" (back then, I thought I had a 7.5" rear).