This versatile axle from General Motors is a perfect candidate for upgrades and custom applications.  It has seen constant duty since 1970.  The 8.5" GM (8.5" is the size of the ring gear) first showed up in the Camaro and Novas.  It continued to be used primarily in passenger cars until 1982.  GM produced a "half-ton truck" version that eventually replaced the 12-bolt truck axle. This truck version was available starting in 1977, but was put into full production in 1982.   Starting in 1999, the 8.5" was renamed and called a 8.6".  This was primarily due to larger carrier bearings and larger carrier case bearing journal.  All versions are affordable used (auto wrecker); The best part of modifying this axle is the selection of aftermarket parts.  Ad traction with an Auburn Gear Posi, Detroit Locker, Eaton Posi, Truetrac, Spool and Change Gears from 2.73 to 5.13.

    Here is how you can identify an 8.5" at the auto wrecker or salvage yard:  Start with counting the 10-bolts on the inspection cover and matching 10-bolts on the ring gear.  Ring gear bolts are a ¾" socket size, pinion nut at the yoke is 1 ¼" socket size.  Also verify the axle has c-clips.  There were several mid-70's passenger cars that used a bolt-in axle version but unless you are planning on cutting and replacing the housing ends, this variation should be avoided.  8.5" Specifications are:

10-Bolt Cover

Case Breaks are 2.56 / 2.73

Pinion shaft is 1.625" 30-spline

Ring gear bolt torque is 60ft-lbs

Carrier cap bolt torque is 60ft-lbs

Backlash is .006 to .010 thousands

Typical pinion depth shim range is .032 to .038 thousands

8.5" Carrier Bearing Numbers LM501349-bearing and LM501314-race

8.6" Carrier Bearing Numbers LM603049-bearing and LM603012-race