Install help please

What I have is a Nepoleon NZ3000 that I am installing. I removed an old Heatilator "builders box" and I am installing this in its place. The Duravent class A chimney just arrived today so I hope to have that installed soon. What I have not come up with a solution for is how to mount a mantel. For an 8 inch mantel it needs to be mounted 18 inches above the faceplate. At this point all that is there is the metal studs. Obviously the Duro Rock will be installed but I don't want to do that until I know how I am going to install my mantel. My wife and I are still "discussing" what the mantel will exactly be. It will either be a old timber or a decorative wooden mantel. So I just don't think that the metal studs with duro rock will be strong enough to hold it. What have you guys done? BTW The metal stud wall is mounted on the face of the original 2x4 wall that I redid to comply with clearance requirements. The plan is to use manufactured ledge stone as a finish material. All comments are welcome.

Here is a pic of what I took out

That would work great but then I wouldn't have my required clearance to combustables. If you look at the first pic the required clearance to combustables is 2x6 header that is towards the top and behind the metal studs, as well as the jack studs supporting the header on either side. Or would it be ok to run the 2x there as you say?

I have attached the clearances and framing requirements from the manual. now I am thinking that the best way to do this may be to attach some 3/4 plywood to the face of the duro rock where the mantel will be mounted with a steel stud behind it. Any other suggestions would be great.

Well I tried to attach the manual. Can someone tell me how to get a PDF attached to the post? I couldn't find anything on that in the "Learn how to post in the forums"