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.
____________________ Ken Clark
real men don't use instructions. they are only
the manufacturers opinion on how to put the thing together!
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.