[longhornchris04]

Short answer: A cat converter really won't do a lot for you... not without improved fuel injection. You may be able to set it right at idle (using your O2 sensor) but w/o feedback, this setting will drift.

Long answer: A catalytic converter only works (or rather works as intended) if the air-fuel mixture is in a very narrow band, much narrower than the engine will actually operate. This was a major factor in the adoption of fuel injection over carburetors. Its also why bosch (and others) developed O2 sensors and feedback systems.

While the 78 & 79 SCs had/have cats on them, they only worked well if the CIS was properly adjusted. The later CIS, Mototronic, and everything afterward use O2 sensors to ensure the mixture is within the limits under idle and light operating conditions.

2-Way cat converters are functional over a decent range of AFRs. 3-Way cat converters require the AFR to be between 14.6 and 14.8. Your idle AFRs are much richer. Good for power but bad for emissions & fumes.