Hollowing out your cats is A LOT more work than it's worth, bro... Check here, Corral, and Ebay for used offroad midpipes. As Harold said, you could find one for around $50 used. I've ran an o/r x on my 5.0 with stock headers/catback, and the sound was real cool. Smooth, louder but not obnoxiously so, and didn't have interior vibrations.