A 360 does not have enough weight in the counter weights to counteract the weight of the pistons and rods so the factory threw a small amount of weight on the converter to help counterbalance the extra weight, the weight is a specific amount and all you need to do is add this weight to your converter, 360 converters already have this extra weight on them or you can add it to your 318 converter, mopar sells a kit with the weights and a way to locate them on your converter, no big deal really and no need to remove the crank, just weld the weight to the converter where it says to.

318s did not have the need for extra weight so there converters are neutraly balanced, no extra weight needed.