318/340 cranks ???

They do interchange, however, the #1 & 2 and #7&8 journels (318) do not have the 5/8th hole drilled thru it to compensate for additional weight of the 340 rod.
This in theory makes the 318 crank stronger.
It may need to be rebalanced internally, due to this.
I used to run the 318-3 truck steel cranks for the added strenghth.