You should get the kind that places the O2 sensor as close to the original location as possible. Sensor placement in relationship to the cat is very important to how the computer determines mixture control both up and downstream of the cat. I've seen driveabilty problems in vehicles occur in non-MB vehicles that had the O2 sensor mounted 1-2" away from "stock".

If the style/price of cat doesn't have the sensor hole where you need it, you can always purchase and weld in an O2 bung in the desired location.

It's also important to note that some amount of backpressure (restriction) in an exhaust system is desireable.

Have you actually checked your cats to see if they're clogged/restricted? I know it's common lore that big pipes and no restrictions means extra HP, however there are many more factors at play than most people realize when it comes to optimizing the power output of an engine. Granted, 20 year old cats are not likely to be in great shape (especially if your engine has ever run rich), and universal ones are generally cheaper, but you should know what you're paying for.

If you really want to "make more power" you should get a little dyno time and see for yourself what modifications/improvements actually work. One of the instructors whose lectures I pay out of pocket to attend has a friend who owns a custom muffler shop and says that about half the time the new $3k system they install actually puts out the same or less HP than the stock one - but the sound is exactly what the customer is looking (and paying) for.

Besides, the biggest enemy in an exhaust system is turbulence - you can do what you like downstream, but until you ditch the exhaust manifold and switch to some properly designed headers, you're really just changing the way your exhaust sounds, not how much HP you're making.

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A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.