Marauders run four cats (two on each side). I have the Kooks system, which replaces everything with two cats total. There are four O2 sensors. Two are located in the header collectors; two behind the cats.
As for NJ inspection: It is *technically* illegal to modify ANY emissions control equipment on your car unless it is in need of repair and you're replacing the damaged/malfunctioning parts. That equipment includes (but is not limited to): catalytic converters, smog pumps, O2 sensors, etc...
If you take the car to an NJ DMV inspection facility, they do do a quick check of the underside of your car. They use a mirror mounted on a pole to look under your ride. As long as they see cats, you shouldn't have a problem. The employees at the stations aren't the brightest.
'03 300b - all the bolt-ons (SOLD)
'93 Mustang Notch - restored w/ all the bolt-ons (SOLD)
'97 F350 Powerstroke 5 spd, 4" straight pipe, 5" stacks, Rockford system, 35" tires...(SOLD)
'95 Lightning - bolt-ons and susp (Totalled)
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'01 Lightning - 3"/4" drop and full bolt-ons... FAST.