On my '76, the white wire in the picture is attached to the hot post on the starter, which should have a cable from the positive battery terminal on it as well. From the positive battery terminal, I also have a white wire that goes into a connector and comes out as a red wire. This red wire meets up with another red wire coming from a similar connector on the negative battery terminal...
I think the white wire is spliced into two, then again into four white with red stripes that feed the fusibles.
I also think that we do have an external voltage regulator.
Anyway, congrats on your purchase and I hope this helps!
P.S. Nice battery tray.
P.P.S. If you don't have it already, download "The Fuel Injection Bible" (280Z specific) and the '76 FSM.