That should be the first thing to check. At about 2,300 RPM the alternator is putting out better than sixty percent of it's rated power in amps and if the battery is discharged it should be pegging 15.0-15.3 volts. If it isn't charging (above 12.4 Volts) then your light is working properly and the system isn't charging.
On these old DN and early SI alternators they were limited in amperage so you can use a 60 Amp amp-meter to test the circuit (just not the 10 amp one in your multi-meter). Modern one wire CS and later alternators out put 105 to 180 Amps just off idle and that is why GM dropped amp meters in the dash in favor of volt meters (they're cheaper).
As to what wire goes where; especially on a modified vehicle I have no idea. That is why I refer to wiring diagrams. The oldest Chevy I currently own is a 1977 Nova, and that is far newer than points style mechanical voltage regulators, so I am working off of fifty year old memories of my 1953 Chevy 150 sedan with a 409 under the hood.