The 1970 Rupps used one like the fox bikes did, Jim Kise makes a repro, Maybe shoot him an email to see what the measurements are on them. Had you asked a couple of weeks ago I could have given you an answer as I had someones 1970 Rupp apart here, but its all done and gone now.
Heres the Rupp one Jim has for the Rupps (and yes they are for 1/2" axles and 5/8" bearings):
#15263 WHEEL BEARING SPACER 1970 all except CHOPPER
Actual Fox part number for there part is 203-611 if anyone reading this has something like above sitting a bag on a shelf and been trying to figure out what the hell its for....Never know :shrug:
Worst case use 1/2" ID bearings and make your own spacer out of 1/2" id tubing, but not as strong a setup.