Those hubs look like they might be the same as my '65 SRW truck. IIRC the SRW hubs like that had a thicker "spacer" cast into them to make the WMS-WMS width wider, so you may not have clearance problems with slightly wider wheels.
I'd be interested in seeing a picture of the '70 hubs; I don't think I've seen or heard of any like that; assuming you are talking about the flat vertical face of the drum being where it bolts to the hub like it sounds. 1/2 ton rear ends got wider sometime in '70, so it might be a wider form of SRW hub or something.
Glad to help another guy out, no problem!
Both my 61 and 64 came with the factory 18" dually wheels so I figured the hubs were original and they both have dually front hubs. Here is a pic of the 70 dually hub. The wheel stud area is about 4.5" sticking out from the drum.
70 hub
