Toss the Datsun alt... They can never make the amp's, and you'll pay too much for it. I put a GM 80 amp. on my 73/240Z.
The GM can be wired for a Datsun voltage meter w/ charging light, as well as, the Datsun AMP meter. My biggest problem was the bracket. It has to be made form scratch. Wiring is straight forward... main batt. feed, and ignition turn on.