Thanks for the picture. The difference in the '70 isn't the hub; that's all part of the brake drum, which is certainly much different than the earlier ones I have.
I'm not sure why the rear hubs on your DRW trucks look like my SRW hubs; maybe something weird GM did? I'd say it was a year-model thing, but if your '61 looks the same that doesn't make sense. Maybe someone swapped them with another junkyard assembly when the brake drums needed to be replaced later on?
I don't have them apart to look at now, but IIRC my SRW had the WMS cast further out like that; presumably to change the WMS-WMS width and keep the load distributed as intended over the inner/outer bearings, but using all the same parts otherwise (or that's what I thought/theorized). Maybe someone swapped them for more tire clearance, in that case? Hard to say, I guess.
I'm interested to hear if the 28177's work. Scrape all the dirt off the edge of the drums and snug them up good with a couple lug nuts when you check; it's a VERY tight fit, and if it's not flat/tight to the mounting surface or it's caked in grime it'll touch. It's even less than an 1/8" gap; actually probably more like a 1/16" of a gap when bolted down and looking through the hand holes. Tight!
Some of my crap:
1963 C-30 flatbed dually 292/SM420
1965 C-10 LWB fleet 250/column shift 3 speed
1965 C-30 9' stepside 250/SM420/4.10 gears (my DD)
Website I made for my crap:
http://www.66submarine.com