There were 60 & 72 decks for the 855 (and maybe a 50"?). I have a 72. I assume you mean mid-mount. I believe you may be looking for M060HDA, however you'll also need the mounting pieces as well as correct PTO shaft. I'd watch CL as well as eBay if you aren't already.
Also, if your site permits, you may consider getting a rear mount. While you cannot mow as tightly around trees, you can cover quite a bit of ground quickly plus if you go to sell it, the buyer needs no specific tractor in order to use it.