The way I did mine was I ordered a flange to fit the exhaust pipes that come to the catalytic converter from ATR, ($20) then cut the downpipe to size and weld the flange to the end of the downpipe. So I don't have a cat. converter, just a custom dp with attached racepipe. The ATR DP should be a few inches longer than where it would meet the actual exhaust system. Later on I am going to add some kind of dump pipe where the cat was.
How ATR recommends you to do it is cut the dp to where the cat. would come to, then weld the flange they provide to you in your downpipe box(kinda hard to find, its wrapped up in bubblewrap), then just bolt it up. Hopefully you have a welder at home or a muffler shop close by because this is easier to do on the car, its a pain in the butt to take that pipe in and out without the passenger side header and crossover pipe on. But the car isn't extremely loud like a open header big block with the downpipe open. Just try to avoid Johny Law on your way the the muffler shop!
