hey before you go bolting the heads back on.
put your o2 sensor in the rear header, then lay your header in the back, then put the head on the engine, then pull the header to the head studs and nut it on there. then mark the fire wall where the o2 is close to hitting. unbolt the header remove the head and header, then dent the fire wall in. you will never have this much room to work in to clearance the o2.