When it comes to the 273 ...
When ordering custom head gasket bore size...note the chamber are usually a hair bigger than the actuall 3.65 bore possibly because they were already in the works for the 318la predecessor... either way the chambers are already bigger than the bore by a notable amount and a gasket too close to the actual 3.65 bore size could hang into the chamber once installed and act like a step/stair case into the bore.
So measure the chamber width across the valve center, they are closer to 3.8" +inches