My ATR uppipe has always been an issue. It was a hassle to seal when I had stock headers, and has been worse since the Poston headers.
What I did (thanks to Herb's suggestion, since my brain wasnt working as quick at the time) was to use a donut (crush) gasket between the uppipe and header. It works. You just need to match up one the right size. I searched through a box of them at Auto Zone and found a couple to try. I know I have the part # for the one that I went with somewhere, but Im really not sure where right now.