You could try a thin metal Oring style exhaust gasket. I couldnt get a leak free seal on the drivers side using the same headers until I put one of those gaskets there. Autozone has them.
I never had any luck sealing a stock size pipe to aftermarket headers.
Make sure the stock pipe didn't crush the ball socket on the headers (which i have done).
Try loosening and or disconneting all for corners of the H-pipe to isolate it from the headers and rest of the exhaust system. Then, re-tighten the problem side and see if ya can get it to seal properly in its correct position. You can even throw an air line up your exhaust pipe that is open as your doing it to see if your having an impact as you tighten up the connection. If you can seal it up, then tighten the other connection on the other header and listen for leaks. then, you can go back and and tighten the rest of the exhaust system (downward side) of the h-pipe. Sometimes catback or even stock replacement exhaust systems move things around a bit. By isolating this out of the equation, it might work because I experienced with my BBK's but I got them to seal pretty good with no problems. Had a set of Pypes and nothing would work so they went back to Pypes and then China I guess where they were made. Also check your H-pipe to make sure it is not torqued at the connection into the transmission crossmember from hitting something. Obviuosly this would have to be fixed and straightened out first to get a proper seal. And finally, does your H-pipe slide freely into the transmission crossmember mounts. With age and time, they can get obstructed from rubber breakdown which does not allow the h-pipe to pull as far forward as it could. Hope these ideas/thoughts help you!