454 Stock? Exhaust and O2 Sensor

Have a 94 K2500 with a fairly rusting underside to include the oxygen sensor. While I have not removed and tested the sensor, the exhaust odor and mileage (11-12 mpg unloaded) indicate the engine may be running rich. Question number 1 is "what is a failing oxygen sensor more likely to cause, rich or lean mixture?".

Question number 2 is something I've noticed while under the truck. The muffler comes first. Conventional wisdom, and every other vehicle I've ever been underneath, tells me that the cat comes first, to treat hot exhaust gases, then the muffler further towards the tail. Is there EVER a scenario where the muffler and cat would be in reverse locations?

The check engine light comes on from time to time. I don't yet have a code reader, but perhaps there is a connection.

You didn't say how many miles are on the Burb or O2 sensor. I believe they are generally regarded as having a 60k life expectancy but usually last much longer. I do believe they tend to go rich when defective.

As to the order of the cat, it was explained to me that the cat could not take the direct heat from the 454, so the muffler was used first to lower the heat.

Awesome, that makes some sense - I've heard elsewhere that the 454 puts out a ton of heat, relatively speaking.

The O2 sensor is original as far as I know, and the vehicle has about 135k miles on it.

Thanks for input!