1973 has some wiring differences, I think, That's why they only spec the kit to 1972.
One option for you would be to figure out what's wrong first. The manual has some tests you can run, Engine Electrical, after you confirm that your meter works correctly. If the VR is working, the replacement alternators will all be 60 amp anyway, so no "upgrade" with a ZX alternator. Everything is 60 amp, unless you convert to a different model IR alternator.
http://www.nicoclub.com/FSM/280z/