New member rear differential width question

[4X4GMC1972]

I recently purchased a 1972 GMC 3/4 ton 4x4. I don't believe the rear differential is original to the truck. It is a Dana 60 that I suspect came out of a Ford although it has Delco-Moraine drum brake assemblies. Where the wheels face up to the drums it measures 65" . I believe that I had read in a previous thread that the width should be 66-3/4" for my truck. Is this correct measuring across the drum flanges? If so I will probably install longer studs and make 7/8" spacers for each side to make it the proper track width. Responses will be appreciated.