904 trans

A 1970 340 should have a steel crank, so it should be a neutral (internal) balance same as a 318. 360's were all external balance, and as I mentioned before the 360 external balance is different than the cast crank 340 external balance. So regardless of the 340 balance, either internal or external, the 360 balance is different. The transmission itself will swap, but you'll have to sort out the balance of the flex plate and convertor to match your 340.