318's all have a neutral balance cast or forged. There are only two basic factory connecting rods. Small (318,273) and large (340,360) in the later years they changed to press pin and I think they eventually started using large rods in all the motors. I would not use a 340 cast crank to help balance a rotating assembly because they are not that common and you will have a hard time if you want to use an after market damper in the future. I think you wont need to add weight to your crank because new parts are lighter. An FYI wiseco/ K1 has a clearance on H beam rods for real cheap they are lighter stronger etc. You probably can't rebuild factory rods for the price that they are selling them for.