Help!! Exhaust Leak

[1991GT]

After 2 weeks, a returned set of headers and tailpipes that didnt fit, I finally got my exhaust on my car (BBK 1 5/8 ceramic unequal shorties, BBK h-pipe w/ cats, and flowmaster 2 chambers). Well its sounds sick as hell BUT I have a wonderful exhaust leak that I don't know what to do about. I'm almost positive its coming from the right side header where it bolts up to the h-pipe. the alignment didnt look too swift when I was installing it but i thought the flange would straighten itself out as I tightened it. Also the car feels much slower and feels like I lost power all throughout the rpm range, which I could understand some torque loss because everything is bigger diameter than stock, but not like this. Maybe the leak is causing the power loss?? Also sounds like its misfiring too but I think that may just be the leak that I'm hearing. I did trim the header flange with a carbide cutter to 2 1/2in diameter because it came from the factory at only 2 1/4in and I wanted everything to match. I put black silicone around the flange to try and make it seal better in case I got a leak like this. I also disconnected the battery for a day or so to clear the computer. I only drove it for 8 miles so maybe the computer hasn't re-learned everything yet. I really don't know what else to do...tomorrow I'm going to try and unbolt the header flange and re-align it and hopefully that will fix the problem. Please give me some ideas, I'm desperate!! The leak just makes an otherwise beautiful sounding exhaust sound like crap and the car doesnt feel fast anymore. Help!!