Catalytic convertors are the best available way to reduce tailpipe emissions. Using a basic chemical conversion process with a mesh screen to trap (some) pollutants and burn them before they leave the vehicle.

Hopefully you've also checked your air filter and pcv filter and/or valve if so equipped, as well as checking your spark plugs, wires, etc, as well as (on an older vehicle - even your gas cap - believe it or not). I lived in sunny Southern California for 25 years, and wondered sometimes if it was sunny and clear there because of the smog (not really I knew the truth).

I do question why the Cat was not on the truck all the time. If it was for "performance" you could purchase an aftermarket, high-flow catalytic convertor (make sure it's made for truck use) and it will perform as well as any without. Late model vehicles are designed to perform well with the equipment they have, partially to discourage people from tearing it out. While I used to tear the air injection pump off of every vehicle I owned, (because this type of "smog equipment" didn't actually perform a useful function - simply blowing more air into the exhaust to dilute it), I wouldn't dare touch the emissions related equipment on late models as it actually performs a function.

Believe it or not, my last streetbike had catalytic convertor inserts in the tailpipes from the factory. (what a joke really - a 750cc pollutes SO much - especially for the projected 5,000 miles a year I would be riding it). After tuning the bike, correcting the carburator jetting for sea level (which the dealer never did - they are set for 2,000 feet above at the factory) I was able to WASTE current 900cc bikes of the time - including the Triumph Speed Triple, all retaining the quiet stock dual exhaust and my catalytic convertors.

Out here in Missouri it is typical for the locals (rural area) to take a brand new $40,000 truck and take the ENTIRE exhaust system off and repipe it with only tailpipes. These knuckleheads drive their trucks with no muffler, no catalytic convertors, and annoy the living hell out of all who come with 1,000 feet of their vehicle. Every time my ears are assaulted by one of these idiots I'd like to install a potato in each pipe for my listening enjoyment. Typical Harley Davidson riders are another topic for another time, but totally along the same lines (apparently when you go deaf in your old age, you have to remove the mufflers to make sure the piece of junk started).

Good call on re-installing your cat, and I hope you pass the emissions test - those little stinkers used to drive me nuts. And when you're done - make sure you're out of the way of the diesel busses and trucks, or you'll be seeing stars sitting in traffic LOL.