Best way? Clean those bolts up real nice with a wire wheel. Go buy some nice big nuts and grind the zinc plating off the top of them. Put them over the bolts, zinc-free side up. Fill that nut with weld, and cap it with weld nicely. Surface area is your friend.
Wire wheel the other undamaged end of the bolt for good measure.
Now, using a torch or induction heater, heat cycle that ****. As it’s cooling, use a good penetrating oil. I like deep creep, but it is $$$. Heat cycle it a bunch of times to work the bolt loose from the corrosion. Then give it one last good soaking with the oil and let it sit. Overnight? A few days? Whatever.
Now, start working it. I like to start by hand. Try alternately tightening (yes tightening) and loosening the bolt. Back and forth, a few degrees at a time. As you make progress, add pen oil. Eventually, you should be able to get it loose enough to just start cranking it out without having to wiggle as much.
If that doesn’t work, you can try using an impact. It’s a dicey move, but sometimes it works.
If it ends up snapping off again, then I would consider carefully drilling. Bringing it to a machine shop may be the best play at this point, as they can fixture it and carefully and accurately drill it out.
