You ask "How would I go about hooking THAT up?
If you choose the Delco 10SI family of so called three wire units and its Neg ground 12 volts
Big rear main output wires to the Load (NOT battery/starter supply) side of the ammeter if it has one....and Id use at least 10 better yet 8 gauge wire.
Small side 1R terminal can (among other possibilities) get its initial excitation voltage from
Feed to the ignition coils input terminal which becomes hot when ignition switch is turned on with an in line diode to prevent run on if ign is turned off,,,,,,,,,,,,,or an in line series resistor,,,,,,,,,,,or a small incandescent tail/marker indicator lamp,,,,,,,,,,,or a combination lamp and resistor so it still works if the light burns out. Theres other possibilities if needed but the coil or ignition switch output is often used for initial alternator excitation.
You ask "It has an idiot light already in the dash so I suppose I could use that instead of the diode.
YES a small incandescent light (if wired correct) can work to prevent run on just like a diode or resistor would
You ask "But how about the rest of the connections?"
The alternators small 2F side terminal is for voltage sense and it can work if jumped direct down to the big rear output stud or better up to the battery itself.
Or you can use a one wire unit with its output up to the ammeter if it has one