IMO the best answer depends on how much tire clearance you have to spare at the chain stay. If there's a tone of room you can crimp the fender in so it drops between the stays, and then you could use an angle bracket to secure it from beneath the bridge.
If there's no room to spare, end the fender at the top of the chainstays (not touching or you'll get noise and paint damage) and secure with an angle bracket
IMO, you don't want to snip the reinforcing ribs off right where the stress from the mounting bracket will be highest. I don't know if it would make a functional difference or not, but I prefer to err on the side of added structural support if there's no penalty.
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