Delco externally regulated alternator

I have a Perkins 4-108 in my sailboat. I wanted to upgrade the alternator from a Delco (12si, I think) to an externally regulated unit of about 100 amps. I obtained a Balmar ARS-4 multi-stage regulator and I was all set to replace the delco with that, and a Leece-Neville alternator(2" delco mount). But I ran into problems: the L-N had a different pulley mounting distance and also (according to the alternator shop), would not work properly with my tach. So the shop recommended that they build me a new Delco 100 amp with a dummy reg. so that the external Balmar would work. INSTEAD, they rebuilt my old Delco (with an expensive 100 amp stator) and sent me on my way.

I was not pleased (the old delco was to be my spare) but I installed it any way. But it's performance sucks. It does not show any inclination to start charging until about 1,400 engine rpms (works out to about 2,900 alternator rpms). If I reduce rpms, the voltage falls off quickly to battery voltage. I went through the system (full field test etc.) and I think the problem is the alternator. So I went back to the shop and asked them to re-test it. I watch and notice that their "test bench" (home-made junk) spins about 4,000 rpm. No wonder they got 100 amps! But they claim they did what I asked (BULL) and so I have to be satisfied with the result.

My question(s) are;
Has anyone else been able to successfully regulate a Delco 12si externally? Or do all delco 12si refuse to charge below 2,600-2,900 alternator rpms?