The better option would be to cut the 2x8, and secure it to the vertical studs. A couple lag screws into each stud, and the 2x8 should be a permanent fixture. Some folks prefer to use plywood for this, instead of 2x lumber.
Since you don't know what the purpose of the horizontal members, or how they're attached to the studs. It's better not to use them to support anything.
Make sure to drill pilot holes through the 2x8, and put the screws in the center of the studs edge. This will prevent the wood from splitting, and give you the best hold. Cutting the board a bit longer than needed, will reduce the chances of the board splitting.
You'll also want to use some fairly large washers on the lag screws, so they spread the load against the 2x8s face. This will prevent the screw heads from digging into the wood too much.