I own a 1991 Camaro Z28 with a dual catalytic converter set up. I have a stock 350 tuned port engine. I would like to know what causes catalytic converter biscuits to disintegrate. Is it true a rich running engine could cause it? Second, what is the best aftermarket cat out there?

Typically unburnt fuel, either from a misfiring cylinder or just generally over-rich condition. The fuel is then burnt in the converter, which overheats it. You really need to fix whatever is causing this before replacing the converters.

"Best" is pretty subjective. If you mean one that won't do this with the rich condition, there isn't one.

Mine chunked up from doing donuts in the snow. Hot converter + snow bank = bye bye converter