You have an isolated field alt. The Blue wire provides 12v to one field terminal. The green provides a variable ground from the VR.
the test bypasses the cars electrical system specifically this path...
battery to starter relay lug stud
to fusible link
to bulkhead
To ammeter
To main splice
To ignition switch
to bulkhead
to ballast resister and alternator.
I read 67dart273s comment and yep I missed the target.
The power to the VR needs to be bypassed too, to eliminate the cars wiring.
All of that path could cause a voltage drop.
By going from battery to terminal on alt you eliminated any posable voltage drop.
Since you are still over charging the issue must be the alt or the VR or grounds