The 8.5 is a 10-bolt axle, the 12 bolt has a ring gear size of 8.875 (or 8-7/8) inches.
Everything on your current axle would have to be VERY CAREFULLY welded on to the older leaf sprung axle you are inquiring about.
Quite a bit of work, it CAN be done, BUT you could very well end up warping the housing without the proper precautions to alleviate warping.