[1969 GMC]

14 bolt or Eaton are roughly equal in strength, I think either one of those would be good for you.

I would avoid a FF D60 rear simply because any aftermarket traction aide is going to be more costly, not to mention the other two axles are a bit beefier than the D60 (bigger pinion, ring gear, axle shafts, bearings).

I personally have an Eaton that is under my GMC, not original though. It has 5.13 gears and a factory No-Spin. My only problem with it was that it wasn't wide enough (65" wms) and I couldn't find any wider rear axles locally, so I have ordered a set of wheel spacers for the rear.