here is a pic of a stock oem 273 commando 10:5 compression steele shim head gasket.... you can see the gasket is egg shaped toward the top near the valley ... could this be an issue with the way the chamber is shaped and the gasket I used. just say n . the composite gaskets I have been using are perfectly round.
Because no one makes 273 specific gaskets, you may need to contact Cometic and have them make you a pair with the 273 sized bore. . A few here have done that. Double check the deck and head flatness when you have those heads off.