I did a conversion like that before . Just get an older voltage regualtor and wire it in, say from a 73-76 or somewhere in there. You can buy the reg and the plug at Mc Parts.
The alternator was computer controlled on those. I have bypassed them many a time when the computer quit regulating the field, to keep from having to replace the computer.
Blue wire (power with key on) to the blue wire at the reg and "t" it to one of the field connectors on the alternator. The green form the regulator hooks to the other field connection. Be sure the reg is grounded before you hook the battery back up
Fact is stranger than Fiction