WTB 68-71 340 forged crankshaft

My machine shop tells me there are some differences between the 318 cranks and the 340 not just the early ones for the converter size and he can tell the difference just by looking at them. He also states you should use a 340 crank with a 340 for balancing purposes and i believe he also stated other reasons but its been years ago. I have one for sure and maybe two 340 ones i am going to sell. They have already been maged and size checked and tagged i just have to dig up the paper work on them. PM me for more info
Thanks