DTCs are error codes. If your catalytic converter doesn't work similarly to a stock converter, the car may throw a code. Get a PCM that has the rear O2 error code erased, and you are good to go. Here'* the PCM for you:
Intense Racing reprogrammed drop-in PCM
Fill in the blanks appropriately and then for the little bubbles:
• Standard PCM
• Stock idle
• Adjust cooling fan speeds to work with a 180º thermostat
• Go with either a stock timing table or the second one from the top if you have 93 regularly available.
• For the DTCs, you can either erase the bottom 3 out of the 4 or just the rear O2. Depends on your budget.
• Leave WOT shift points at stock
And now that you've got the PCM, you can go to a smaller pulley. You have to do that in that order for the mods to make sense. Also, drop in the PCM and then take it easy on the car. It takes about 2 minutes to put in the new one because it'* located behind the driver'* side headlight, but it takes about 2 days of normal driving for it to learn long term fuel trims. Without that learning period, your car will not at first run properly at wide-open throttle.
Modular Pulley System. I suggest going with a 3.6" for now, but it doesn't matter whose pulley you buy.
ZZ Performance modular pulley kit
Intense Racing modular pulley kit
A Magnaflow cat in either 2.5" or 3" is available for about $60 + shipping from ZZ Performance. It'* the cheapest by far but is also highly recommended. It feels like it'* not there, but you're still cleaning your CO emissions to CO2. It may save the Earth from spontaneous combustion. Or not. I like running without a cat, but you didn't hear that from me.
ZZP Magnaflow high flow cat
Intense fender well intake that works with Bonnevilles
Also, get a 180º thermostat to work with your new PCM. It will give you a tiny bit more power by letting your engine run 15 degrees cooler so you can have a wee bit more dense air. The recommended Autolite 605 spark plugs should work just fine. And then you're set for a while before some heavier modding.