Burning would be a tricky business in this image.
I would flash the paper and print one grade higher.
This way you would have a uniform sky, where instead burning would almost surely too evident in some areas.
Great Looking rock.
If you have the Ansel Adams book "the print" you should check what he writes about it.
There is no way I can tell you how much the flashing exposure will be because that depends by many factors: f stop, distance of the light source from the paper, diffusing material etc.
I did a quick search and this is nwhat I found http://www.apug.org/forums/archive/index.php/t-23139.html
That appears to be a cloud formation directly behind the the highest point of the rock, so you only have the right hand side to burn in . Give it a go and see what you get. That pre-flashing technique will drive you around the bend.