THis is the way I did it for a friend. We pulled the cat, busted up the honeycomb and hollowed it out. Got a length of 4" pipe, trimmed the cat so that the pipe slid right through and left about an inch or so (i think a bit longer) sticking out of each side. Welded the pipe in place, then fitted it back to exhaust system and welded it up. So we ended up with straight pipe through the cat. Looks original, worked great.
Im not sure but it seems to me that hollowing out the cat and welding back the "shell" would cause some type of turbulent issue since the exhaust enters into an "echo chamber" of sorts and loses velocity, as opposed to the straight pipe method, which would maintain exhaust velocity. At this point this is just an opinion on my part. Anyone care to chime in an shed a little light on this hypothesis?