Are 340’s rare

Look at a 318 piston side by side with a 340 piston, before you make a statement like "318 and 340 bore size difference isn't much"

Besides the numbers matching guys out there, THE reason the 340 is and will always be king, is because of that "minor" extra 0.130" bore. It's much more than just the displacement, it's also the headflow that accompanies that bigger bore.