best rear disc brake conversion kit ?

Doctor Diff kits are pretty good and they can utilize factory tapered bearings unlike most other kits. Give Cass a call. He's a great person to work with.

10.7" kit: http://www.doctordiff.com/rear-disc-brake-kit-10.7-rotor.html

11.7" kit: http://www.doctordiff.com/rear-disc-brake-kit-11.7-rotor.html