OK, two issues. Gauge does not work and something wrong with charging system.
Charging:
1. Don't buy any more alternators unless internally regulated.
2. Clean the battery posts and the cable clamps. Check cables all the way to other ends. Inspect and clean terminations at starter and at ground point. Be sure this chrome alt is grounded to it's brackets and the bracket are grounded. No paint or rust or grease or corrosion on any ground points anywhere. The regulator also needs to have a good ground.
3. Test voltage regulator per Jerry. I don't get into that because I recommend internally regulated ALT. (Delco 10si)
4. How are you specifically testing the alternator output? Tell me how, where and when. If I tell folks how to do things, they say "yeah, that's how I did it".

Ammeter Gauge:
1. Is gauge wired per my post above? Exactly or not? The ammeter gauge wiring is different than the indicator light style, so you say the harness "matched up fine". That just means the connectors pugged in "just fine". BUT the connectors are wired differently with different colors and wires in/out of them go to different places. So check per my post...carefully...with neg battery cable disconnected. Follow wires visually or do continuity tests with both ends of tested wires diconnected from any components. This is to confirm that ammeter is wired correctly. Report findings all around.
2. Does gauge needle jump or move at all when key goes on? or while starting?
3. Does it move any when headlights go on?
4. It does sound like an ammeter. That is what Chevy used in the needle gauge clusters.
5. Is your voltage regulator marked F 2 3 4

These two issues may or may not be related. Be sure ALL the wires discussed here are checked for attachment/terminal tighness, corrosion and general condition.
Be very specific on all your answers.
Check your post more often

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn