I have an '04 STi with a stock exhaust and just bought a used Invidia downpipe. What kind of gasket do I need to make this downpipe fit the stock exhaust well?

Am I supposed to use the donut between the aftermarket downpipe and rear exhaust or just a flat 3" gasket? I needed something temporarily so I fabricated a ghetto gasket but I think its leaking.

Also, should I reuse the bolts with the springs, or buy replacements without springs?

Thanks for any help!
Dan