Í've used JB weld on all kinda stuff ... it's great! BUT ... frankly I would not use it to attach O2 bung to exhaust ... I don't think it'd hold up to the heat. I suggest go get it welded.
BTW, I saw a post here in last coupla weeks where there was problem in O2 location because the AFTERMARKET O2 was too far back from motor ... not hot enough ... was re-relocated forward ... seem to recall fellow CACC member 69myway (Chris) may've posted it ... he is installing aftermarket digital fuel injection in 69 BB. You might email Chris on this ... he may have invaluable insight on optimizing location for O2 bung.