atlee has one to put 850s on a 12 but you need 18 gear for their STC. It's an easy field approval to write up based on already approved 800s and all Bushwheel sizes being approved. Use the bushwheels install check list for the continued maintenance part of the FA.
I did a field approval for 850s ona 12 wirh original gear on N3969B when i owned it. you might check with the new owner out of Boise for a copy or request one from the feds