273 guys..Heads up!!!!

Sounds like a surface issue..even if the gasket is exposed it should be able to take the combustion pressure .
On Vic's 273 before I did any combustion chamber work I scribed the bore on the cyl head as to not make the chamber too large..
With that being said even if the gasket was exposed it shouldn't fail unless the combustion gasses are getting inbetween the head and block or there is a whole bunch of exposed gasket like .250 ..
It's definitely a head scratcher, but the first place I'd look is the surfaces.
That little 273 that was built for Vic has 175 lbs cranking pressure and goes to almost 7000 rpm and he's been trying to murder that engine for years now.
No issues,not even a valve adjustment yet..that little 273 makes some serious onions...