Exhaust leak,Between header n cat..

[hothatch]

Hi everyone, I have a problem with my exhaust again. I recently purchased a new apexi noir exhuast and new magnaflow cat. I put it on and the flex area between the strup shorty header and magnaflow cat is leaking and its making my smooth ep3 sound like a ups delivery truck. Does anyone know what size donut gasket or whatever goes on the end a strup shorty header to a magnaflow cat? The gasket that came with the magnaflow cat does not fit and that is the same as the oem gasket which is too small .Please help, thank you