This what you describe is completely normal charging system behaviour. The ammeter shows you current flow into ("C") and out of ("D") the battery.
The stock '60-'69 roundback grounded-field (or "single field") alternator came from the factory in variants ranging from 27A to 60A. The one-year-only '69 Imperial roundback alternator for use with a 3-pin electronic regulator came in one rating only, 60A. The '70-'71 roundback isolated-field alternator for use with a 2-pin electronic regulator came in ratings from 30A to 60A. The squareback ('72+) isolated-field alternator for use with the 2-pin electronic regulator came in variants ranging from 46A to 78A. These alternators, even when new, didn't produce anywhere near their max output at engine idle speed. Now, forty-odd years later, there are additional problems: the wiring and connections are old and have higher resistance than when they were new. The alternator has very likely been replaced, probably with a thrown-together "remanufactured" unit that has even poorer low-rpm charging than the original. The voltage regulator is also old and no longer as quick or efficient as when it was new. The whole electrical system relies on grounds through body sheetmetal; these were (barely) okeh when new but they're no longer new. The ignition switch, headlamp switch, high/low beam switch, wiper switch, and ammeter (through which all the current in the car passes) are old and have high internal resistance. All of these factors add up to create the low-output-at-idle issue.
Before you toss a higher-amp alternator on your car, see
here (including links) and
here. Also run a ground loop of 12ga wire from the alternator housing to the voltage regulator base, and from the voltage regulator base to the battery negative terminal.
The alternator you're after for the easiest, least-costly upgrade is from an '88-'91 Dodge Dakota, D-series truck, or B-series van with a 3.9 V6, 5.2 V8, or 5.9 V8; or an '88-'89 M-body (Diplomat, Gran Fury, New Yorker Fifth Avenue, or Canadian Caravelle). Does not matter if it's the
Chrysler unit, the
Bosch unit (shown w/serpentine-belt pulley), or the
Nippondenso unit (shown w/serpentine-belt pulley); all three are physically and electrically interchangeable. The Nippondenso is the best design, also the smallest and lightest -- it's the one that looks like a ¾-scale Mopar alternator, with the rounded, finned housing halves. Any of these three is much more efficient and has much better low-RPM output than any of the older Chrysler-designed alternators. They come in two ratings: 40/90A and 50/120A. The first figure is max output at engine idle speed; the second is max output regardless of speed. In the '88-'91 applications listed above, they have the two-groove V-belt pulley you'll need, and electrically it'll hook up to any earlier Mopar regulator. You can use the '67-style regulator (though please make it a Standard-BlueStreak VR-128 or a NAPA Echlin VR-1001 solid-state electronic item) by simply connecting your car's one and only existing field wire to one or the other of the new alternator's field terminals and grounding the other alternator field terminal. Or, you can go to the '70-up electronic regulator and use both of the new alternator's field terminals. Shave 1/8" off the front of the existing "lower" (pivot, not upper slider) bracket.
Don't get a "remanufactured" alternator from a parts store; they're junk. Also avoid the aftermarket "100% new!" items, which are Chinese copycat garbage.
Other options: there are
these bracket kits to put the Toyota version of the Nippondenso alternator on. That's functionally fine, though I don't prefer it because the Toyota alternators are internal-regulator items. It means if the regulator fails you are dead in the water until you replace the alternator. With an external regulator you can replace just the regulator. More of a philosophical objection than a practical issue; the internal regulators (and the external ones, for that matter) seem to hold up okeh over the long haul.
Also, put in
headlamp relays.
