I used to manage and install at 2 Midas shops here , I've installed and custom bent literally hundreds if not thousands of systems.
Wherever you have a joint , it needs to be tightened by an impact gun , this is the only way that the seams will seal properly , you can't properly pull a clamp down and dimple the pipes so that they seal with a ratchet or wrench. The quickness and torque of the impact causes the clamps to pull right and uniform. When it's done by hand they just slowly pull tight and do not "massage" the two pipes together or dimple them properly. Also when done by hand , the clamps can't be run as tight before they want to gall or snap , as they can when the impact zips them up all at once. At somepoint you've probably tried to back off a new clamp that was just tightened and had it snap , that's because the clamp pulls the saddle out of shape and the threads gall as they get tight , this happens far before the clamp has actually sealed if done by hand. I've seen plenty of cars come in for car shows and when Bloomington Gold was here where the people installed the exhaust at home and then had it twist loose and come unadjusted and start rattling everywhere because they installed it at home with a 9/16 ratchet. There really is a trick to installing exhaust correctly.
Anyway , here's what you do. Install the exhaust , tighten the clamps by hand , NOT overly tight , just enough that it won't fall off while driving to the muffler shop , because he'll have to re-adjust it after you get there , don't get excited if he wants to cut your clamps off so he can install so he can get a nice fresh pull on them. Also when you flat-back exhaust (install it on your back , on a creeper) you can't get the proper perspective of hanging the pipes correctly , at a muffler shop they'll have screw jacks , etc. so that if a pipe is a little low they can push it up while tightening it with the impact , this happens so fast that it'll lock it together exactly where he wants it. It's all how you hold the pipes while hitting them with the impact. So hang it toether , tighten it a little , so that you feel it won't come apart , and then take it to them. DO NOT WELD IT , there is NO cause to EVER weld a system or a joint unless the pipes weren't bent right , or the joint was over-swedged where it fits over the other one. Let an experienced installer adjust it , clamp it , and check for potential rattles. Also he'll turn your clamps right and cut the excess off with a torch so you don't have those nasty looking threads hanging out under the car (also people who install it themselves usually leave these threads hanging down and don't cut them off , then they catch on a driveway or something in the road and pull the exhaust off from there back). It won't cost you hardly anything , they'll get it way up in the car so it doesn't look like "monkey bars" hanging down when you look at the car from the side at a distance , and you can forget about your exhaust problems until it rusts out and needs new.