ls400/celsior custom duel exhaust

[Armed_N'_Hammered]

hey im looking to get a full custom exhaust for my celsior i was thinking of going with s&s headers, magnaflow hiflow cats, into duel 2.25in pipes with a resonator per side and magnaflow straight through mufflers. but i was wondering if i should put an x pipe in and if i can just use the old O2 sensors. and has anyone had problems with their stock cats i think one might be clogged it smells too hot and could it be causing my o/d light to flash on and off if the o2 sensor is getting a bad reading? i guess thats off topic of custom exhaust but any help and thoughts would be good.
thanks.