Okay so i just bought 2 rear cats for my 2001 mustang v6. My question is what is the whole in the middle of the cats. The cats are magnaflow high flow cats #99055HM. I think thats were the O2 sensors are supposed to go, but not to sure. On my stock exhausrt the right cats the O2 sensor is on the front of the rear cat and on the right side the O2 sensor is behind the Precat. Please help me out! i want to install them but dont want to mess anything up.