You definitely need to weld the arch.

Cut the arch lip off about an inch or so up from where it is now and weld the two skins back together to restore the strength to the shell.

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tbh I would remove the wide arches to do it properly.

Personally I wouldn't have tigersealed them onto the existing paint anyway, I would have taken it to metal, laid up a layer of fibreglass mat over the metal and then glassed the arches on to that.

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Kind of hard to explain without pictures

(I'm on my phone) but that's the

"right" way to do it.

Bond/rivets is great for over fenders that people use on S body nissans etc where they're not blended in but if you just bond/rivet then filler over the top it's likely to crack