speed,
This isn't a SI alternator...correct. The regulator you have replaced, the one you also had tested, (?) is a external unit mounted on the firewall or inner fender...correct.
I am not understanding what the connection is between the radio and this problem. It seems you are using the radio to help diagnose this charging system glitch.
The voltage you measured is obviously way to high. The voltage regulators purpose is to limit this value. It seems like two units have been unable to do this properly, so either both were funked up, or the root problem remains.
My first thought would not be a ground issue, but rather a question dealing with the alternators ability to 'see' system voltage and respond accordingly.
If the first regulator is still at hand, it would be interesting to attempt to adjust the voltage relay to see if you could tune it to cutout at the proper voltage.
Did you ask your auto electric person if there could be a internal problem with the alternator itself that might cause this phenomenon.
A '57 GMC is a negative ground rig isn't it?
Stuart
