I built a 1000 sf one room addition to my house for my Trophy Room. The side walls are 9' high and the center ceiling is 14' high so it doesn't fit your 8' restriction, but here are some things that you should consider:
Make the room much bigger than you think that you will ever need. (I never thought that I would ever fill my Trophy Room, now both it and my Living room are full.)
Put at least 1/2" plywood on the walls, under the sheetrock so you can hang smaller (deer size) mounts anywhere.
Make a floorplan of where the wall studs are so you can easily find them to mount larger (elk size) mounts on them.
Minimize windows. Direct sunlight ruins mounts.
Paint or surface the walls with a color that is lighter than the mounts so the mounts stand out.
Provide sufficient lighting, like ceiling track lights, so each animal receives light from both sides.