More info needed:
Does it have an alternator conversion? GENERATORS are RPM-sensitive, so you'll see an increase in voltage as the engine revs. Not so much for ALTERNATORS.
Is yours an internally or externally-regulated alternator (most are internally regulated nowadays).
Does it use a sense wire (are there 3 wires coming off the alternator, or just one?
As for the wire gauge to be used, that depends on the amps the wire will be carrying, the length of the wire and the acceptable voltage drop.
--Mark
1973 SIII 109 RHD 2.5NA Diesel
0-54mph in just under 11.5 minutes
(9.7 minutes now that she's a 3-door).