I used Copper RTV and so far so good. I have an A&C Kit and the headers were 1-3/4" OD and the sidepipes were 2" ID. I had reducers welded into the side pipes and tabs welded onto both the sidepipes and headers at diagonally opposing pipes on each side. The slip fit was very tight and after bolting the tabs together I just ran a bead of the Copper around the joint and forced it into the joint with my finger.
Below are two pics of the setup before I assembled them.
Good Luck,
Tony
