[Scott Douglas]

I'm not an engineer.
What kind of mast is it on now?
Aluminum?
Will steel interfere with the antenna function?
What is the wall thickness of the current mast pipe it's on?

Looking at your picture, based on my experience building stuff, I'd move your clamps further apart so that they are above and below the flanges of the beam. IMO this would give you a better mount because it would add a degree of twist resistance via the flanges of the clamps to beam flange interface.