.......I replaced mine with the MagnaFlow FWIW. Bought the 2 1/2" in/out size. More because these were CARB/OBD2 approved. If that is not a issue for you something like a Catco would be much cheaper
If you want DIRECT replacement cats for your B-body, the Magnaflows and the Catcos will fill the bill.
They may not be CARB compliant, but they are visually indistinguishable from the OEM cats. I used them when I had SLP shorty headers.....and with a 224/230 cam in a 383ci LT1, I passed Georgia emissions with no problem.
Now.....if you REALLY want hi-flo, performance cats, then you can go that route....but your muffler shop will be doing a bit of fabbing and welding.....and you will pick up close to nothing versus going with the Magnaflows or Catcos.
We use a very free flowing converter core. They are the spiral wound metal matrix. We can sell you cores with ends only as we no longer stock the factory manifold flange. Or if you choose to go with our headers, we can build the hi-flow cats to work with them.