If you want you could simply put a flare seat in the O ring flat end with a suitably sized drill bit...a 3/8 countersink comes to mind...
but seriously,
the flare inside the radiator is factory made in case the auto manufacturer wants to place a pipe with an inverted flare direct into the radiator...
The bung is flat to seal against the hydraulic seat on the fitting...Graingers have them, as to Earls Plumbing etc...
If you have a decent speed shop...not a parts store...near you I would give them a try.
I have ten here as I use them all the time on projects...
