They put the same head on the 318 and 273 when they were made the same years (67-69). The 273 and 318 heads are completely interchangeable as they both have the smaller ports and 1.7/1.5 valves.
The 340/360 heads have bigger valves and will work on the 318/273, but will require a notch in the bore for valve clearance for the 273 because of it's smaller bore.
The early heads have smaller combustion chambers and may help raise compression on the later 318's.