Some of the things that cause this are.....
Voltage drop between battery and voltage regulator power (ignition) terminal
Poor connections and voltage drop in the same path as above, to the alternator output post
Poor ground between VR and battery
Poor brushes or other alternator problems
bad VR
One thing you can do is scare up some wire, even temporary, even solid, like "romex" (house wiring) At least no14 for all but output of alternator at least no 12 or maybe two or three 14's wired in parallel for the output.
Try Bolting a ground wire on the VR mounting bolt and to an engine block bolt
Try a jumper wire from alternator output post to as close to the battery as you can get. The starter relay "big stud" if nothing else
Try a jumper from battery (or starter relay) to VR IGN terminal
No particular order, any one or more can cause this.
If none of this helps, gamble on whatever's cheapest, a new VR or maybe pull the alternator and check it out.