A 273 is 80% the size of a 340 if you look at the port cc and cfm there on par with there cid ratio with each other in other words a 273 head is about 80% the cfm and cc as a 340 X head.
A 318 is 93% the size of a 340 but basically comes with 273 head a j head is more inline with what a performance 318 should come with.
No one considers the 273 and or 340 be over headed.
P.S. If you hold the 360 to the same ratio it should of came with something like Edelbrock Rpm heads. Probably why the 273/340 are considered the revver's of the family.