what is the best alternator and voltage regulator replacement brand

You may also consider having your original alternator rebuilt. Many rebuilds these day are poor. First one out the box didn't even spin before I left the store. Second one works, but always sounded a little noisy. My original has a very internitant short . Yours being a '68 if original would also have the single wire field with a mechanical regulator. Might consider upgrading to the isolated field with newer regualtor. Requires some mods, but not too complicated. Also might consider upgrading the ammeter wiring.