There are many kinds of alternator setup. The two kinds you're probably thinking of are the '60-'69 "single field" system and the '70-up "dual field" system. Either setup can be made to work well and both setups are reliable, but the '70-up system is more advanced and there would not be a good reason to remove it and install the earlier system in its place. Neither of these systems is particularly good at supplying a lot of charging current at idle and low engine speeds, so if you need a lot of electric power at low engine RPMs you would want to look at using a different type of alternator. Amperage rating selection depends on the maximum total electrical load your car will place on the charging system, which is determined by what kinds of electrically-operated equipment you have on the car. In stock form, these cars don't need much alternator capacity. Start adding big stereos, big lights, high-draw ignition systems, electric fuel pumps, backglass defoggers, etc., and you need more.
Just tossing a high-amperage alternator in is inviting trouble and damage; the stock charging circuit will not happily cope with a bigger alternator for long -- main charge wire, bulkhead disconnect, ammeter...if you're lucky the damage will be confined there. If you're not as lucky you'll have a fire.
Upgrading the charging circuit is a very good idea even if you're not putting in a big alternator, though if your stock wiring and ammeter are in good condition, you're generally safe if you keep it below 50 amps; higher than that and the existing wiring will not tolerate it. Do not get a "remanufactured" alternator; they're junk. You can get a brand-new Chrysler 46-amp alternator from Old Car Parts Northwest, 206-300-1083. Guy's name is Ray; you can tell him Dan Stern sent you if you like. The ones he has are '70-up type with two field terminals.
Use a good quality regulator, which is getting harder to find these days (too much junk from China). Ray may still have some genuine Mopar US-made regulators.
If you need more than that, swapping to an '88-up alternator as described
here is a good and easy option (the '88-up alternators work directly with the '70-up type regulator; the regulator suggestions in the linked thread apply to the '69 and earlier cars).
For the charging circuit upgrades, there are many options. You can beef up the ammeter, you can replace it by a voltmeter, or you can bypass it entirely.