Winch fasteners...

[Brion Toss]

Hi,
First, I think riv-nuts are better for this job than helicoils, because mast walls are so thin. There's just not enough thread length for happy helicoils. Unfortunately, your holes are in the wrong place, and you are talking about torturing the fasteners in -- hence the question about bronze. I am afraid that, no matter what you use, you'll end up with a bad job. Even if the fasteners can somehow go sideways, the mast is softer than them. You'll end up with stresses and deformations that you don't want, especially on something supporting a winch.
If I had made such a chuckleheaded mistake (and of course that has never happened) I would probably have an aluminum plate made for the area, significantly larger than the winch. I'd put it right over those misplaced holes, fastening it thoroughly, but not where the winch fasteners will go, and then tap the winch fasteners into it.
Really, don't sell yourself and your boat short by making a mistake worse.
Fair leads,
Brion Toss