[drivemusicnow]

... you asked to have the cat hollowed out... but are afraid of emmisions?

I'd suggest getting a resonator put in line on the straight pipe back. depending on the type of resonator, many will eliminate the higher freqs... I'd also go with a straight through, so you don't add any backpressure vs the stebro system, which i believe is actually engineered, like the stock system, to provide a certain back pressure.
glass packs eliminate lower freqs, so this would be a bad idea.
If you want to be able to sell the stebro system again, i'd cut out a section near to the cat, and weld flanges on both sides of the cut. this would allow you to hook up the "test pipe" vs the resonator whenever you choose.