Can a 318 crank and rods be used in a 340?

Yes, the 318 crank will work... but with some caveats; we put a 273 cast crank in our 340. We won't be hammering on it so we felt the cast crank was adequate. You may have a cast or forged 318 crank; the forged will be stronger, but the cast is supposedly good for up to 400 HP or so.

The 318 rods are lighter than the 340 rods and will likely be weaker than the crank but again, it kinda depends on how you are using the engine. If you hammer on it at all, I would use something heavier or better. I am starting to favor the $300 SCAT I beam rods set; balanced right from the factory and good strength for an intermediate power engine with good rod bolts.

The 318 crank balance is set up for lighter pistons and rods than the 340 stock type. But, the exact answer to the new balance situation all depends on the new pistons and rods you use. If you use lighter hypereutectics pistons (like KB's), then the weight combo can end up lighter than the 318, and the shop will end up taking weight off of the crank (easy). If you use heavier 340 pistons like the Speed Pro's then you will end up heavier than the 318 combo; that is expensive to re-balance as you may have to add weight to the crank instead of taking it off.

So to answer your question, yes, you are almost certainly going to need to re-balance. How much that will cost depends on the rod and piston combo that you plan to use and that depends to some degree if you are going to have parts that produce more power than a stock 340. So you need to answer more questions before anyone can give a more complete answer.