For the GN's the 8.5,31 spline axles are good into the 6-700hp range drag racing. They typically don't use the tapered bearing because they aren't concerned about cornering, but that is a solid setup in the 9" rears. Unless you have plans of over 700hp, this should be plenty. Buy a 31 spline trutrac if they make it for the 8.5. If not the eaton 31 spline clutch posi is tough to beat.
Once you get good axles (33 spline ) and a spool in an 8.5' the limit becomes the ring and pinion itself. But that is way up ther in cars drag racing with transbrakes making 900+ hp and weighing 3500lbs.
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