Charging system suddenly quit.
OK, next
Remove the regulator and jumper those two wires together. Just like hooking the field to 12V this should cause full output, but this time you are checking the wiring "end to end"
Also under the following conditions:
With wires jumpered together
and key in "run" but engine off
Check voltage at battery
and then check voltage at your jumpered connector and post them
Start it up, see if it charges as before
By the way the VR MUST be grounded. MUST
A new VR does not mean a "functional" VR
The only thing I can think of if the above test shows OK is.......
you got a defective replacement VR
or the alternator has / had a partial short in the field. You can measure alternator field current draw. Just hook the field to a battery with your multimeter in series, on DC amps
The fields vary over the years, but somewhere between 3 amps and "just under" 6 amps
Turn the pulley with your hand as you measure as the brushes are sort of intermittent