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Without a new outlet flange you can only put a 2.5 inch downpipe on the turbo.
You need to get in touch with ATS to get a new turbo outlet flange for the 3" pipe.
They also are a good place to get the 3" downpipe.
If you are pulling the motor to install the clutch, it would be a great time to fold the body seams over to make room and install the 3" downpipe.