The cats are needed to pass inspections. So, depending on how rigorous the inspections are, you might need the stock cat. I'd find out what you can get away with at inspections and make your decision based on that.
My stock cats were a very restrictive part of a very restrictive exhaust system. I put on hi-flo 2.5" Magnaflow cats but I don't get inspected.
1992 Tbird LX 5.0, 3.73 gears, 1995 Cobra intake, deleted air silencer, 2.5" mandrel bent dual exhaust, SCP shorty headers. Updated four-hole fuel injectors. SK shift kit in AOD. Vogtland 0.9" springs, Tokico and Bilstein shocks.