Are 340’s rare

Icetech said
No one is saying take a 318 over a 340.... yet once again... here we are...

You said it. You said the 318 has a slightly smaller bore. What matters is the bore to stroke ratio. A 318 will never have a B/S ratio of a 340. Neither will a 360.

While the numbers may look small, the ratio matters.

318: 1.180
340: 1.219
360: 1.117
408: 1.007

Two numbers jump out. That’s a big difference.