Not sure if you have this one solved
but thought I'd add my 2 cents worth.
Be careful of going Balmar
external regulator on a Hitachi, I believe the Hitachi is N-Type and the Balmar
reg is only suited to P-Type. Sterling Power have two versions of an external alternator regulator than can be set to either P-Type or N-Type. The Hitachi alternator does need the field wire installed but Sterling give a pretty detailed description of how to do this, a good DIY guy should handle it no problem. Doing a full conversion from N-Type to P-Type as described above is quite a lot of work and it renders the internal regulator (used as backup) useless.
I have recently installed the Balmar MC-614 onto our Electromaax alternator and it certainly makes a difference having a smart-charge system, worth doing.
Here's the instructions of how to install the Sterling Power external alternator regulator. BTW: the basic Sterling unit is about one third the price
of the Balmar unit but with less programmable features.