Another Charging Question

I got 12v with the green field wire disconnected and that post of the alt grounded to the case with a jumper (other field wire still hooked up). No matter what I did be it on/off lights, brakes, in gear, or off idle it remained at 12v. In the voltage difference between the alt and battery acceptable? I've seen the voltage drop as low as 11.5 in gear with the lights on so I upped the idle to get it up a bit higher. That just seemed too low to me for the HEI. If I swap the alt what would be a good upgrade (square back or denso) and with my setup what's the safe Max amp alt?