You can't weld s/s to a titanium coating. You must remove the coating where the weld is gona be. You can try, but this will not make for a good seal. It will be prety ugly too.
You said after the collector. I thought the collector was the end of the line for the header.
Couldn't you put the fitting in the pipe after the header, and before the cat?
Just my $.02 worth. Good luck <img src="/forums/images/graemlins/kewl.gif" alt="" />