I have a 72 gmc with an ammeter and no idiot light that I know of. and am switching to an internally reg'd unit for a '78 chevy truck and have read a dozen threads, and some of them say a complicated mess of explanation about idiot lights or resistors and crap.
Above all I'm following this thread:
http://67-72chevytrucks.com/vboard/s...d.php?t=800471
I do not believe he says to use a resistor.