How to frame this wall

I am going to be finishing the basement with 3 inches of closed cell spray foam, tile floor and mold resistant drywall. I would like to use metal studs because I am concerned about humidity/moisture.

My question is how to frame the wall under the duct in the picture. I have a field stone foundation covered with a skim coat and painted with some type of water proof paint. It is not level. There is only a quater inch between the duct and the foundation. I don't want to frame inside of the duct because I would lose so many square feet.

How would you frame the wall? Can it be done with metal studs?

https://flic.kr/p/FdAe8Y