JDS#,
Denny's giving you good advice on the non cathederal route. My taxidemist designs trophy rooms and says 10 1/2ft - 12ft is best for many reasons. It is easier in long run for all mounting and cleaning tasks. He strongly suggests you use 3/4" plywood behind whatever you have on face of wall. If you do 10 -12 ft you can put plywood on top 4' all way around to save. It's easier in long run to do it all over wall and you'll never have to worry about where to hang stuff.
Denny,
Now that you have all that taxidermy in one place. Have you given any thought to treating mounts to prevent and bug problems. It only takes one bad beetle or even a moth to get in from outside and you have big problems. McKenzie the target guys also have big taxidermy division. They sell a product called PROTECK you buy it and use rag and atomizer to spray on mounts and work into hair on critters with long hair.It's around $30.00 a bottle and is cheap insurance for all the work you have there. It would probably take 3-4 bottles to do all you have there and every other year will do the trick. I have seen some real disasters with whole collections, as most of the time by the time anyone notices the damage is done. There is a company in the US that does travel around and provides a similar solution, but it's cheaper to do it yourself.