If you can find replacement lights, designed for halogen bulbs, that would be the ideal way to go.
Nigel, thank-you for that. Your logical observations align with the conclusion I have drawn from looking at a variety of different experiences when people have tried to enhance their lighting on classic cars.
I already have doubts about the efficiency of the modern replacement headlamp units that most of us have fitted. The prism contruction of the lenses is suspect and the silvering of the bowls is no better than the originals, which was not long-lived in any case.
It would be ideal if I could locate some much better units designed for halogens, but I doubt such a thing exists at the moment. So I am considering attaching rally-style, auxilliary headlamps during the winter months. As these have modern, computer-designed reflectors and are obviously fitted with halogens and operated by relay, direct from the battery, I am pretty sure I will be able to buzz along at top speed on unlit roads.
This modification won't be to many peoples' taste, nor would many people have the need for it. It might be more acceptable if I fitted authentic auxilliary lamps of the period, but then I would be likely to meet with the original problem. I have some compact, modern lamps in mind so maybe we will soon see how ugy I can make the front of a 500!