For your time frame, I would go with cable, the crane should have a similar rigging set up for the jib, as shown in the photo and any I have come across in the "COWANS SHELDON" book I have.
I will leave the rivet detail up to you.
CS had a couple of different set ups to support the jib on the match truck. From A frame timber to steel supports.
Looks like it came with cable (string) on the original box so perhaps stick with that. Almost certainly make operation smoother. Nice truck combination but I wonder if the main carrier will need a counterweight.