I found an alternative windshield wiper arm that works on your Datsun Z car. I share it with you in this video. Don't pay a ridiculous price for used ones.
I was hoping to see their sweep and how much window coverage they had if different since you mentioned the one that is not the same length. BTW awesome find and video
I'm not 100% sure but I believe that the outer part that I was bending is stainless, however, It is slightly magnetic which some stainless is. The other part that rivets to it may be chromed steel but it also looks more like stainless and is a little more magnetic than the outer part is. They look to be pretty good quality.