Steve,  I  believe that the 854 would have a similar cab to the JD 9660 that I ran EZ-Steer with.  Here is a previous thread on AgTalk with pictures.

http://talk.newagtalk.com/forums/thread-view.asp?tid=116695&mid=826336#M826336

By mounting the 500 up high I would not use the on screen button for engaging the EZ-Steer, use either a footswitch or button on the armrest.