Diffusors eat light. And loads of it. I would go with fresnels.

In my experience it's always better to have a bit wider flash coverage than angle of view, at least with hammerhead flashes, the bottom of the frame tends to get less flash exposure otherwise, IME.

A 28 mm FL on 35 mm or 65 mm FL on 6x9 lens is approx 75 degrees, 60 should cover it, shouldn't it? So that 60 degrees must be wrong.