Is there just one single heavy wire attached to the back or is there also one or two more smaller wires coming out and connected to other posts on the alternator?
Then it's probably a Delco 12SI. There's not much you can do to increase the output voltage. The one-wire alternators based on the Ford 3G have an external voltage sense wire (IIRC yellow) attached to the output terminal. You can extend that wire and move it to the (+) battery cable so that it more accurately senses the system voltage, minus the voltage drop in the alternator charge wire.
