The alt has 2 field terminals right? (blue/green wires-70 up system). Pull the green wire connector off of the alt field terminal & at that stage the alt is recieving fire via the blue field wire & if charging then the field circuit is being grounded (full fielded) inside the alt to the case which includes the green wire brush/brush holder. Good visual on/in the alt to see what's grounded that shouldn't be. Pull both field terminals & see if either male field terminal has continuity to ground (should not) & might see how many ohms you have between the 2 field terminals them selves as you spin the alt by hand (there's a spec for that) but I think you'll find something wonky before you get to that. With the alt field terminals plugged back in if it's full fielding with the reg connector plugged in & still is when you pull the reg connector then either the alt is grounded as previously described or the green field wire between alt/reg is grounded somewhere between the alt & the reg. EDIT until we solve it if you have to drive it pull both field terminals off of the alt so that it DOES NOT overcharge. running the batt down 1 time like that (which is no different than leaving your headlights on for a bit which ain't a dealbreaker) is FAR less damaging (to the batt) than what full fielding will do (to the batt & the rest of the system) and when fixed do charge the batt back up with an outside battery charger rather than letting the car do it as you drive