Where to buy correct alternator for a 1969 340 swinger non a/c car

Old Car Parts Northwest has shelves full of new old stock Chrysler alternators. You ought to be able to get whatever amperage rating you want, within a few amps. See here for detailed info on charging circuit upgrades. If you're actually driving this car, upgrading to a carefully-chosen electronic voltage regulator is a wise thing to do. There are several ways to do it; you can rework the alternator control (field) wiring to go to the '70-up regulator (Old Car Parts Northwest also has a good stock of NOS Chrysler ones), or you can keep your existing field wiring and use an electronic regulator designed as a drop-in for the old points type. As with everything else, they come in a range of quality; I prefer this one.