I thought I found the answer….but no

Ground one of the fields then you won't have to run a newer VR...
I would rather see him run the 70 and later VR. They are much easier to come by and a better design.

But if you do modify it, just ground the terminal to the alternator case. BUT BEFORE YOU DO please do as I asked to make sure one is not already grounded, AKA check continuity from each terminal to the case. Both should be OPEN to the case.

You don't know how to check continuity? You don't have a multimeter? Test lamp?

If one of the field terminals is grounded, and you ground the OTHER one, you will burn up a 69 VR. If one of the two is grounded, and you use a 70/ later VR, and if the wrong field is connected, you will burn up the switched ignition "run" circuit. What I am saying is, it is COMMON to find an isolated field alternator--such as you bought--with one terminal grounded.