X heads on a 273??
About 20 years ago "Mopar Action" did a write up of a 67(?) 273 Dart call "Darn Dart": it went 10.80's . "X" heads, TRW pop ups, MP .509 cam, I don't remember carb or intake. Yes, they twisted the liven daylights out of it, that was the point!
It was some place in the Virginia/West Virginia area.
So it's been done, boaderline practicle; just a matter of what are you trying to accomplish?