10-bolt plates will work, but you may have to grind the center axle bearing holes oversize a tad. I have a new Moser 12-bolt under my car and I used the backing plates from an old 10-bolt. I don't know of the Moser axle bearings are bigger than standard GM or not because I used C-clip eliminators. But the holes only need to be enlarged slightly. I used a carbide bit in a grinder and it's a snap. Hope this helps.
Pretty sure 10 and 12 bolt axles use the same wheel bearings and backing plates, but Moser are making 12 bolts and strongly suggesting they supply them using big Ford Torino press-on bearings and retainer plates.. Thay are a far better and safer way of retaining the axles as opposed to C-Clips, and C-Clip eliminators are still a no-no for the street. http://www.moserengineering.com/moser/faq/
Anyway, the Torino bearings are 3.150" OD as opposed to tiny 12 Bolt 2.255" OD so the backing plates need to be opened up to ~3.200" so the bearing will pass through when installing. Much easier and neater done on a lathe if you have access to one, which is what I had done.