A couple thoughts:
1 - The OEM 6 volt headlamp bulbs were rated at 24 watts (= about 4 amps). Your 36 watt bulbs require about 6 amps eac). So if you add the amps for the head/rear light, then another 3 or 4 amps for the ignition you are looking at a constant 20 amp load.
2 - The stock 6 volt generator is good for a maximum of 16 - 17 amps. And this is with following:
- the generator brushes/commutator in top shape
- both the fan and generator pulleys sheaves set a far open as they can go without slipping (to spin the generator as fast as possible)
- the generator 3rd brush (if equipped) set to "max output"
- the engine spinning at governed RPM
- the generator cold (maximum output drops by a couple amps as the generator warms up).
If any of the above are not met, generator output will be lower.
Check both the generator AND fan belts. If they are at all loose/stretched, replace with new. And run them with the adjustable sheaves as far OPEN as possible.
If the generator has an adjustable 3rd brush, set it as close as possible to the nearest fixed brush. (Note later production generators is not adjustable - it is permanently fixed in the maximum output position.)
You might try swapping the VR with the one from your M and see if it holds the load better than the one on it now.
Incidentally my experience growing up running H's and M's at night is they will constantly discharge several amps except running wide open. However with a good battery they would run like this all night and still have enough reserve to crank up the engine the next day.