There are two types of alternators. If yours is original to the year of the truck, it also has a regulator, usually mounted somewhere on the radiator core support, with 4 wires in a plastic plug connected to it. If the regulator isn't controlling the alternator properly, you'll get no charge voltage. The other type of alternator, from about 1972-forward, has an internal regulator. The wiring is different, but it still requires the correct hookup.
I'd suggest you find a shop with a good electrical specialist and get them to do a quick diagnosis. You can throw parts at the problem forever with no success, particularly if there's a wiring issue to deal with.
Jerry
