An update for anyone reading this in the future. I took the ring gear to a very competent machine shop where a friend offered to tap the ring gear with me. The result? Even with the highest quality taps and a proper jig to align everything it is absolutely NOT Possible  to cut threads in a hardened ring gear. I ended up making a bucking tool and pressing the rivets with a hydraulic press. It was not nearly as difficult as I thought it would be. 

Also the the gears are well made and were easy to set up and get a good pattern. The car drives great and the rear end is quiet. A big Thanks to Larry for thinking this up and Steve at the filling station for digging through his old stock and finding a set for me.