Clamps are going to swivel around and stuff no matter how much you tighten them down, it's annoying - except between the cat and muffler, always use a clamp there so you have an easier time pulling it off to install a new muffler+tailpipe without wrecking your cat next time. Put it where you want it, clamp it, drive it to a muffler shop and have one of the guys weld it up for $20, pull the clamps if you want. I have a dinky little fluxcore welder so I just do my own exhaust now.