External voltage regulator to internal voltage regulator?

If you have a firewall regulator you do not have a "one wire" alternator Is this the original Mopar setup? Then you have an alternator with one field connection. Modern replacement regulators are almost all or all solid state. There is no advantage to an internal (integral) VR. Mopar never made such a thing. There are some aftermarket add-ons, the only ones I've seen are NOT internal, they are a small box that bolts to the rear of the alternator

Now try asking the real question?

What are you doing and why are you trying to do it?