I've seen that butt crack thing before... The outer layer pf the twin waled exhaust tube has rusted enough to yield to the pressure from a muffler clamp. The inner tube just folds over like this.
I have had good success painting exhaust tubing. I used an aluminum heat paint. Just be sure to degrease the tubes if you want to paint. And if you weld on, full wled or just a tack, remember to crawl back under with the heat paint rattle can and hit the welds.
It's still going to rust, but will in fact take longer. Or you can clean up rusty shadows that will eventually return and hit with the paint. Depends how much you love the underside of your truck!