I'm not sure that having the holes close to the top edge would be much of a problem. You may need to grind a bit off the side of the blind nuts if you are using those, and reinforce the edge of the firewall.
I would be a bit leary of drilling new holes as they may be weaker if not drilled perfectly, and bending force will be greater as you move to the center. I can't remember why, but I used a set of composite fiber mounts that i had on hand. I guess I thought the metal mounts looked more brittle. Whatever you decide I think will work.