Your profile states "west coast", which I'll interpret as the Republik of CA. In CA, you can put any cat on your car as long as that specific cat's application sheet lists YOUR car. The Emissions Twits can look up the model number and check to see if it has you car listed as one of its applications.

The rules absolutely vary by state, so before you purchase, call the Emissions Twits to see what they require.

Here in Nevada, we're going to cut the exhaust pipe just below the "Y", weld on a flange, then down to a high-flow cat, weld on an O2 sensor bung at the cat outlet, then a single pipe out to a muffler in the back. If I can find some Euro manifolds, then it will be duals out the back.

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There is a very fine line between "hobby" and "mental illness."
FrankenBenz: '87 560SEC with an '02 cammed LS6 and custom 4L60E.
Bling: '87 560SEC, ported/polished, lowered
Lisa: '87 560SEL, lowered