The bigger pistons and rods is why you can't just use a 318 cast crank to make it internal balance. As you stated the 318 crank is lighter so it matches the lighter pistons and rods. If it was as easy as putting a 318 cast crank in to make it internal then Mopar would have done it. A 340 cast crank and balancer are special and can't be swapped for other cast cranks or balancers unless some balancing work is done. I have a '73 340 that I put a '65 poly 318 crank in and it required some metal to be added to the counterweights because the poly 318 pistons and rods are lighter than the 340 stuff. I even used the lighter SIR rods and still had to add metal.
Chuck