A stamped backing plate with all of the bends is much stronger than anything in a "flat plate". I'd think 1/4" in a flat plate would suffice. I know the Wilwoods I just put on the Duster are about 1/4" and are aluminum.
Scratchin it definitely sounds like you have 11" backing plates...now you just need to determine which shoes and drums are correct. The magazine add above may be the way to determine that.
I'm not positive from the pics but it sure looks like a deep recess from the axle flange to the back of the plate. Could be 11x2.5 or maybe even 11x3...