deep dish brake drums?

[buickracing]

Hi folks,
a new guy here.. I just acquired my grandfathers original '68 GMC C-10 (camper special?).. I've been improving/upgrading it including installing a fresh 327 I had, rebuilt TH-400 with shift kit, completely rebuilt the drum brakes.. which leads to my conundrum. (pun)..
The rear drums are the standard 6-lug, 2inch by 11 inch standard on most 1/2 tons as far as I can tell. The problem is that MY drums have an axle-mounting flange that is roughly 1/2 deeper than every single replacement drum I find. (i.e., if you lay the drum flat on a table, the center of the drum height is 3.5 inches instead of the usual 3 inches.. In other words, my axle shafts appear to be 1/2 inch longer than stock, and the drums therefor have a 1/2 inch bell shape which correctly locates the drum surface over the two-inch shoes. Its killing me.. I've gone through many websites thinking that perhaps the dealer installed a rear-end from a different truck (k-10, 2-20, suburban, van.. etc.) .. but none of them seem to be like the rear end I have (leaf-spring, 10-bolt, 3:55 rear)..etc.. has anyone else seen this? Or know what is going on here? The thought of getting custom axles, or a new rear end because I can't find a couple of $25 drums is too much!.
Thanks in advance,
Barton