340 heads on a 318?

+1 to the above...

360 heads will perform just as good on a 318 but....you must have them shaved to get the compression up or the engine will be a complete dog.
If they are shaved properly, they run fantastic on a 318. You also gain larger
valves by using the 360 head. The 2.02 valves on a 340 head are complete overkill on a 318 unless you are running it constantly in the higher rpm's. The larger valves can actually hurt your low end.

Lots of people say to use the mid 80's 318 head (302 casting) with portwork.
I have heard this works well but I have always went with tinkered 360 heads.
Doing this, you are still stuck with the puny 1.78/1.50 valves.

It's really a trade off between flow and compression ratio.