If you're talking about "undercharging" being the negative deflection of the needle at base idle, at night, I don't think anything you can do will change that. My '66 did that when new (we bought it when it had 7000 miles on it). NOT specific to fuselage cars, per se.
Do a charging system check, FIRST. See what it's really putting out at idle. I seem to believe that the production pulley is the smaller one, anyway. I have seen larger ones, but not smaller ones that I know of. Police/taxi units had larger alternators anyway.
Find an "old, embedded" electrical shop. One that was around when the Chrysler alternator systems were on normal, used Mopars. The person running such a shop should know how to rebuild and test them, AND probably do it more reasonably than one of the "modern" shops you might have inquired at. I concur, things are easy to change in them, other than the front bearing. Brushes are a normal rebuild part. Diodes only if they fail (let juice go the wrong way in the circuit), which could require a soldering iron.
The other side of the deal is that for the price of a refurbished alternator repair, you could purchase a reman unit from an auto supply chain, or RockAuto, or similar. With a warranty, except it'll probably be one of the newer versions, with better cooling and such. Maybe even a few more amps?
Find that "old shop", as I mentioned, and you'll probably find somebody that can do a decent job of rebuilding what you have. It'll take some looking, as they will probably be "off the beaten path" somewhat, many times. AND look for an older, embedded auto supply while you're at it, who might be able to refer you to somebody that's been around for a long time, in the vehicle electrical business. Maybe even an auto supply where they still have real books! Using the computer for inventory controls rather than parts look-ups on the older vehicles.
Remember, too, that Chryslers were not designed by GM or Ford, so there will be some quirks they have that the others do not. And vice versa. Some things that Chryslers do that GMs did not do, or not in the same way. Each manufacturer had their own design orientations and priorities back then.
By the way, check the bulkhead connector for the "10 gauge red wire) that goes from the engine compartment to the ignition switch. Check for corrosion at this point. ALSO, make sure that both the battery terminals and the cable ends (+ and ground cables) are clean and bright. On a modern system, a little corrosion at that point can drop effective alternator output by 10%. In other words, make sure that all charging system connections are clean and tight! AND that the hot base idle speed is to spec.
Happy hunting!
CBODY67