john theres a place between the exhaust header and the primary cat just past the flange . you will need to drop your exhaust and plug the exhaust so the filings dont drop into the cat. drill a hole and weld the bung then insert the O2 sensor
you could also unbolt the flange and slide a piece of sheetmetal in between. drill the exhaust header just before the primary O2 sensor and drill and weld the bung. then pry down on the exhaust to allow the filings to slide off the sheetmetal. thats how i did mine