SBP disc won't fit on hub

I am converting my 68 dart from 10" drums to SBP. I wanted to keep my original 14x4.5 wheels. I obtained used hubs and spindles from a guy in Los Angeles and the machined areas on the hubs all fit well on the spindles. I got all the other parts and installed spindle, etc. I now find the machined area in the discs is too small to slide all the way on to the hubs. Diameter across the hubs is 5 11/16" and the machined area on the disc backsides is approx. 5 3/16". Does anybody have specs on the SBP hub? I was told by the seller they were not Centric discs, neither are they Wagner as they seem to not available. Got any answers? I'm going crazy!