If I understand you correctly, your pulling your CAT/flex head pipe off your car. Then your going to weld increasing transitions to go to possibly 3"? If so, just unbolt the V-band clamp at the head pipe, pull the side sills and seperate the assembly from the rest of the exhaust. Clean the exhaust inside and out at your cut point. I also used a soft wheel on my grinder to ensure the weld area was contaminant free. If your using stainless, you should back gas while TiG welding or at least apply solar flux to the back side/innerside of the weldment. The factory service manual calls for V-band replacement after removal. I used anti-sieze on the inside of the V-band and re-torqued to factory specs. with no leaks. If you don't use anti-sieze, the V-band tends to tighten non-uniform and will probably leak. Make sure you don't get anti-sieze inside the flanges, as some anti-sieze products make contaminate the oxygen sensors.
