Alternator wiring

I can't tell from the photo which alternator you have. The early ones have a round back, and were rated ~30A max though one could upgrade the stator windings to get ~60A and some probably came that way from the factory (police, etc). Later ones (~1972+) have a square-back with 3 diodes you can see hanging in the airflow. Those were used thru the 1980's. I had one on my 82 Aries which liked to vaporize its diodes once per year like clockwork. I think those were rated ~70A, and more importantly work better at low rpm.

Regardless, the MBRP400100CT diode is rated 200A in each leg, so should work for any alternator you install. Of course, the current rating is kind of incidental; diodes are really limited by heat so read the datasheet carefully. Your ammeter can pass an additional ~50A. The smaller Vishay VS-175BGQ045 diodes I put in my Dart are closer to the limit, which provides more signal on the ammeter at increased risk of damage. They were a bit less elegant to install, requiring soldering.

In answer to your final question on ammeter scaling, I don't recall that my 65 Dart's even has numbers on the scale. I recall that later cars had "70 A" or such at max scale. With the diode, it definitely will be a non-linear response, moving similar to factory on the low end and reaching a limit where the diode takes over and passes most additional current. It doesn't matter much to me since I just want to know "charging" or "discharging", without the dreaded "smoking". If I really want to know the alternator current, I have a Fluke clamp-on DC current probe. BTW, the diode in the reverse direction is to protect the ammeter when drawing from the battery. Ideally, the ammeter should be the only path to and from the battery. That is true for cabin loads, but transient underhood loads like horn and starter do not go thru it. Of course, any current going thru these new diodes will also bypass the ammeter.