1966 Barracuda not Charging

Lets get back to "simple" first OK?

Here's what you can/ have

Jumpering power to the field is an excellent step as it eliminates the field wiring to the regulator, and the regulator itself

So what might be?

YOU DO NOT GROUND THE FIELD to test!!! Insteand, Run a jumper from BATTERY PLUS to the alternator field terminal. (Did I misunderstand what you did?)

Make CERTAIN the field is drawing current. Many rebuilds come as an isolated field, IE both brushes insulated, so in that case you must GROUND the field terminal not connected to the regulator (green)

Make certain the alternator output stud is not cut loose from the battery

So:

With engine off, disconnect alternator field wire. Check field continuity to ground. If it is open stop and find out why. If it shows continuity (and if you have an ammeter function on your multimeter) wire your meter IN SERIES with the alternator field to the battery + and measure current draw. Should be at least 2-3A, turn the pulley and see if it changes

If this is OK, jumper the field to battery and run engine at fast idle. Measure battery voltage AND THEN measure voltage at the alternator output stud

If the output stud is "way high" much higher than battery, then charge path to battery is broken

If the output stud is low, about same as battery, it is not charging. Since you have (I hope) verified that the field is drawing current, THEN THE rebuilt alternator IS BAD