Why does flowmaster not list these as compatible with the American thunder cat back exhaust system 17233? Is there something I am missing?

 

one thing to realize is that any exaust company/shop regards the third gen cars in regards to GM original design. dual headers to a single pipe then center in dual out muffler behind the axle. if it is a system that runs dual pipes or a strieght muffler they will say it is not compatable. i took my car to 4 exaust shops here to have true dual glasspacks installed on my firebird and everywhere said it was 100% impossible. took it to a shop next county over and they did it.

 

I'm not exactly sure about the 1991 or 92 T/As but these are for single cat applications.

Here's one difference between these CAT Backs.
The one you're looking at is
for a single 3in. Diameter outlet cat. =
(Notes:
Models with single 3.00 inch flanged catalytic converter.)

The #17234 is a 3in. with a adapter into the a stock cat with 2.5in outlet.