The nuts are welded to the red part on the inside. You could push a smaller screw through it or drill new holes. I have it on a wooden beam and have screwed an aluminium plate (15mm) to the beam and drilled holes where the nuts are. So I first screwed the plate to the beam and then the winch to the place.
I drilled two holes in the aluminium rail, front and back. Then I put washers approx. 1cm underneath and mounted the construction to the wooden beams. Now I can also move the winch horizontally. Just pay attention to the load capacity.