Welp...the D&D exhaust were just about to fall off so I pulled up into the welding shop. NO..I didn't mig weld them on. We slid them back into place easily and re-tightened. I took it for a ride around a country block and pulled back into the shop and tightened the clamp even more. If it doesn't hold now, it's not because it's not tight. I tightened it to within 1/8 turn of stripping it out. I can't believe it's such a small bolt in that clamp. It looked smaller than 1/4". I'm not sure of the size, but it took a 10mm socket. I'm think the V&H pipes had a 3/8" bolt. I guess it could have been a 5/16".
I can hear when it's slipped back. It makes a different noise.
Ken, D&D has another clamp that solved the problem for me. The clamp you have with the 10mm bolt is a pos. The other clamp, funny as it seems, is actually smaller - not quite as wide and the wall thickness is a little less - and it uses a 5mm hex head bolt. May be that it is more flexible, don't know. But it sure solved the problem of the muffler slipping off the header pipe.
Both clamps are made by Five Star Mfg, Crest Hill, IL. The one you have is p/n 27 72MHDH. The one you want is p/n 27 72M. Give it a try. Think it'll solve your problem.
Jerry