As BC59 says you do need a shunt if you aren't going to peg the ammeter needle when you start your truck with a slightly flat battery.
Just connect the two wires from the shunt to the ammeter. then connect the other ammeter wiring to either side of the shunt. Look at
this photo, the two wires go off to the ammeter, the connector block takes the other wiring which would normally go to each side of the ammeter itself. BTW, the shunt in the photo was made for a 20A ammeter on an old Austin car. you need 3 100W 1 ohm resistors in parallel to shunt the ammeter on a chevy truck, I run this setup on my truck with a 68A alternator.
