To the best of my knowledge, ALL 318's cast or forged cranks ARE internally balanced, ALL 340 forged cranks ARE internally balanced, ALL cast 340 cranks ARE externally balanced, therefore you can swap a balancer from a forged 340 crank to a 318 crankshaft cast or forged, you can swap a balancer from a 318 crankshaft, cast or forged to a forged 340 crankshaft.
Hope this sets you straight