The pipes were originally set up to be a push fit into the heads. If the machine has a bit of fretting wear inside the head bores and they have loosened up a bit there are a couple of ways to tighten them. One was the previous mention of expanding the pipe a bit at the joint. The other would be to wrap aluminum or stainless steel foil tape around the pipe in the area that it joins up to shim it. You still don't want them to be too tight in there or there may be trouble removing later. The long bolt that goes between the pipes pulls the two pipes together to aid in clamping the exhaust to the heads. The finned ring that clamps around the pipe and snuggs up against the head also helps to seal the joint a bit better. I've seen old pipes where the little welded on tubes, that the long fixing bolt goes through, have broken off. You will need to repair the pipe if this has happened. I'm not sure if Hitchcock's has the original type pipes for the 57 or not. They have the larger size at the muffler end than at the head end. The 58 & later pipes are all the same size at each end.
Kerby