Recently bought a 1970 Tempest.  A true barn find, (really, it had over an inch of dirt on it) bare bones stock (350, 2bbl auto with no air and radio delete)  Did not realize it for a couple of days, but it does have a posi-trac rear end.  Just curious if this was common on this car or if it may be somewhat rare.  It all looks like it came on the car from the factory, down to the steel tag on the cover bolt about additive. 

 

 

Nice find.  The posi could have been swapped in, but could have been ordered from the factory.  Only way to find out is to check the build # and VIN #.  Don't have the link for you to check this, but I'm sure Frosty would. Check under the rear seat for the original build sheet.  Might still be in the car!