318 crank shaft in a 340 ???

Is the 318 crank forged or cast? 318 is internally balanced and 73 340 externally, as others have already stated. If you are swapping cranks you might as well put some higher comp. pistons in @ the same time. It will need to be balanced anyways. 10:1 would be much better than the 73 340 8.5:1 JMO. Otherwise your not going to gain a thing by just swapping cranks.