There is possibly suitable tachometer pin already in alternator (at least in '90-model) called "P" ("W"), which is connected to the end of field, according to GMC service manual. I said possible, because I haven't tried it yet and don't know if it really works. But anyway FYI . It is in four pin connector where warning light goes.



Connector is that black lump next to Delco, in picture and if I remember correct, P-pin is most upper pin (I don't have service manual or car available right now)