The Dexter specs gave the needed size. Big help.
I installed new Grade 5, 3/8-24 x 1 1/4", bolts with Grade 5 nuts and new lock washers, new loaded backing plates, bearings, races and seals.
It had studs pressed into the old backing plates instead of bolts (from 1964
Machine shop dude measured the drums at "exactly 12 inches", saying "These have never been turned and don't really need it now....had him clean them up just the same. We decided new thinner "China metal" backing plates were probably better off with bolts than pressed studs.
Headed for Alberta Rockies! Now I have real BRAKES!
Thanks for the help.