There are at least 2 variations of 60 rear spindles.  The one that the internet thinks gets weak when you bore it is the early ford 'small' spindle.  This spindle has the same diameter for both the inner and outer bearings. The 'large' spindle has a bigger diameter for the inner bearing.  I ran a bored 'small' spindle housing for a few years.  We just bored it the minimum necessary for the 35 spline shafts to fit through and I never had an issue with it.  The spindles survived the housing breaking in half so I'd say they were OK.  I suppose if you hogged out the 'small' spindle more than necessary it could be considered thin and weaker. The 'large' 60 spindle is the same as a 70 spindle, it just has a smaller bore.  When you bore it out it becomes the same as a 70 spindle.