Hi Patrick..First you have to decide which you want..12volt or 6volt. If you have a regulator that will do either then simply wire it to your chosen configuration. If you have one that will only do one or the other then perhaps your choice has been made for you.
Also there is the question of whether you have the early (45w) output dynamo or the later (60w) dynamo. That will govern the rating of the bulbs you can use at either voltage. At both 6 and 12 volts the tail light will be in the 3-5 watt range. Brake light (if fitted) and horn loads can effectively be ignored as they are intermittent..There are also 'losses in the system' which will vary depending on the quality of the wiring connections (particularly at 6 volt) and the condition of the battery. Not many people realise it but an old and/or badly performing battery can soak up most of the dynamos output...
Finally there is the headlamp bulb..this would be 24 watt at 6 volt on a WD M20, though I have run 30w bulbs on a nicely set up system without problems. At 12volt, on the later dynamo I have run a 35 watt halogen bulb with good results, though on a military bike there is the question of the bulb holder type to consider and what bulbs you can source to fit it....
I find 6 volt lights are quite adequate...particularly if the headlamp reflector has been resilvered (this makes a huge difference) and the focal length of the pre pre focus WD light is correctly set up...
Ultimately though, the arrival of LED clusters for the headlamp will be the thing that really makes a difference...I'm not sure of the current developements there....Ian
email (option): ian@wright52.plus.com