340 Vs 360 which is better?

The great thing about the 340 block is the bigger bore means more cubic inches potential when stroking.

As has been said already 360 is the best bang for the buck and more cubic inches in stock form.
340 and 360 have 4" bores the 318 has a smaller bore.The 340 has a shorter stroke same as the 318 so if one was to stroke a 340 and 360 with the same crank it would be the same cubes.