340 crank in 318?

Actually balancing a 340 crank in a 318 is probably going to be a lot cheaper than trying to balance a 318 crank for a 340. It generally cost a lot more to add weight to the crank than to remove it.

And yes I had to sue national cylinder head for selling me a 318 crank when I asked for a 340 crank a long time ago.

And I agree with the above, unless you are going for a matching numbers engine, spend your money stroking a 360 and grind off the casting marks and paint it like the 318 it should be. No-one will know the difference.