You have a couple of options depending on how much wiring you have and how the IR alternator is wired and where the large output wire from it goes.
If you don't have any wiring for the battery gauge you could just replace it with a volt meter, easy to wire in. Or you can run the output wire from the alternator to a junction in the engine compartment and run a length of 12 gauge wire from there back to the battery positive terminal to charge the battery. Then to get the battery gauge to work you'd just have to run the two black wires to the gauge. One runs from the main junction
where the alternator wire goes and the other would run from the positive battery terminal.
This is how the wiring would be run with the fusible link in the battery positive charge wire and the two black wires with their small 4 amp fuses. The diagram doesn't show the positive battery cable which would go to the starter solenoid.
From your description you may have a one-wire alternator, which won't make any difference if it's wired like the diagram. you just won't have the two smaller wires to the alternator.