My understanding is that the 320's all had generators (not alternators) and some of the early ones were positive ground. The net seems to indicate almost everyone converts them to negative ground, and converting them to either a 411 or 720 alternator is popular. So what could be wrong with yours depends on specifics, whether it's box-stock and if not, what alterations were done. Presuming your battery connections are all good, if it's a generator you can do a quick charging test with a simple bulb-type test lamp. With the truck running, connect it between the generator output terminal and the battery (plus terminal if you're negative-ground, minus if you're positive ground). The lamp shouldn't light. If it does, the generator is sucking juice instead of producing it.