Original Amp Meter External Shunt?

[74Corvette]

I am working on a 71 GMC pickup. When I bought the truck it came with new aftermarket wiring. Wiring works fine but what I am trying to do is get the original amp meter to work. I know the original gauge is an external shunt style meter and as I understand it is basically measuring the voltage drop across the shunt. Problem is I am pretty sure I do not have the shunt anymore. So what exactly is the shunt? After studying the engine bay wiring diagram the only thing that makes sense is the fusible link. The wiring harness I have is a painless system, I don’t know what model, and it has a large main fuse, which I am guessing is the modern version of a slow blowing fusible link. The amp gauge is brand new and functioning. Any ideas on what I can do to wire up the amp meter?