Its always best to use the wideband o2 that will go in place of your upstream o2, or you can weld on another o2 bung to your downpipe, and then just keep it plugged with the proper bolt, unitl its dyno time, then the dyno guys can just screw in their wb o2. The tail pipe wideband is not *quite* as accurate, but its pretty darn close. Not sure if the cat will effect readings.