I just pulled the dash on my 77 project and thought I'd do some headlight circuit testing. This car has the OEM style headlight bulbs. With the Key Off, Turn sign switch and headlight switch fully removed, I get no power to the fuse box as expected. I connected only the Red wire and the White/red wire to the Headlight switch. With the Headlight switch off there is no power to the fuses as expected. Turning the Headlight switch fully on and there is power to the fuses as expected. Checking the power at the round Sub-harness connector of one headlight (in this case the left side) I had power at each of the 3 wires, not expected. I realized the power is routing through the right-side bulb which is still connected. To verify I disconnected the Right-side Sub-harness, this removed power from both the Low & High Beam wires at the sub-harness connectors. I still had 1 wire with power (Red on the right & Red/yellow on the left) in each Sub-harness connector as expected as these wires are coming from each of the two fuses. At this point I reconnected both Sub-harnesses. At the column, in the 6-pin connector, I temporarily grounded the Red/black which activated the Low Beams, I moved over and then grounded the Red/white which activated the High Beams as expected. Keep in mind that I still ONLY had the Red wire and the White/red wire connected to the Headlight switch, no other wires and key was off.
See if you get the same results.
On a side note, I actually had an experience where BOTH headlights were burned out, likely from a voltage regulator issue causing over voltage. Look closely at your bulbs to ensure that you aren't having a similar issue with the High Beam circuit.