My slip fit connectors on my ERA leaked like crazy. I tried just about every sealer known to man and couldn't get them to stop leaking for more than a month or two at which point the sealer either cracked and fell out or burned up. I also tried using an expander to increase the size of the header. That didn't work either. I finally solved the problem by doing this:
I welded a short length of stainless steel tube to the side pipe. I then notched the steel tube with an angle grinder. After slipping the two together, you can tighten up the steel tube causing the sidepipe to clamp down on the header pipe. No more leaks.
Chris