14 bolt has bigger bearings, bigger housing, bigger gears, much better pinion support. With that said, a chromo 35 spline 60 will hold up to anything a stock 4.0 can throw at it. Mine is 30 spline behind a 4.6 stroker and 100:1 crawl ratio on 38's, and I haven't had any problems with it at all.
To make the 60 35 spline you have to bore out the spindles to clearance for the 1.5" OD shafts, and get a 35 spline carrier.
