273 head is the same as 318 in port volume. If you put a 340/360 head on a 273, you’ll lose so much airspeed,compression,and shroud or crash valves etc.
The 273 doesn’t flow enough air to warrant big heads, 170ish cfm (stock) puts the engines peak horsepower potential at just over 6grand.
I was faced with putting a set of J heads on a stock 318 bottom end. An old head mechanic at the time (around 2003) warned against the loss of compression etc.