What year range is the y-pipe from? That's pretty slick.
I'm still fighting the flush face flange between the cats on my 'burb. Going to try the Remflex 2045 gasket. If it doesn't work I'm going custom back from the head pipes, and doing a different muffler setup.
Its a Jegs #31004, for 1999-02 Silverado / Sierra applications.
Remflex is the best, if it leaks after their gasket. You've got something seriously wrong. As thick as their stuff is, its very forgiving.