sb crank numbers?

Hi, I hope someone will enlighten me alittle. I am restoring a 72 340 duster with the numbers matching motor. when I got the motor apart, I found it was a late 72 that had a cast external balance crank. The balancer stated 340 cast crank only on it. The crank number was 3462387. The torque converter had no weights on it. I know the 360s have weights on the converter and the balancer. Should the cast 340s have that also? I was thinking of putting a steel crank in the motor so I pulled the pan on another 340 with a build date of 5-68 to check it (with the thought of using it). It is not steel either, casting number 3751841. Is that one a 318 crank? Are any 318s external balanced? Does a 360 converter work with a cast 340? Maybe I should just use the original crank as it is in good shape. I just thought a steel crank motor might be more desireable since I may sell the car in a year or two. Thanks, Ray