OB,
For a good sized whitetail shoulder mount, I used a 3 1/2" long lag bolt with an eye on it. 5/16" or 3/8" diameter would be better for holding that critter. I cut out about 1/3 of the eye with a hack saw so that it looked like a fish hook, looking from the side view. Just leave enough sticking out from the wall for the hook to be all the way inside the plywood. You can adjust the mount fit to the wall by screwing the lag in or out.
If your mount is wide enough, I would use two for that weight. Find two studs - usually 16" on center. Just drill some new holes in the back of the form wood on the mount to match and hang. Enjoy!
Ray