Hi Julian,
I struggled with the idea of driving lamps like the later Gulf cars used, but how to do it? We're running Cibie or Hella 5.75dia headlamps with the parking lamp included. As your over the pond, check out Europa Spares, they have the lights and mounting pods.
Installing the lamps is quite a fiddle so give yourself lots of time, a pint or two and the support of some good freinds
The lower lamp opening varies in size for different manufacturers so you could be in for a real fight. Once they're done it's worth the trouble.
With each lamp having 1)Parking Lamp, 1)High Beam, 1)Low Beam there's lots of combinations of optimising illumination and functionality. What we decided on was: full time amber parking lights, high beam for turn signal and low beam for added light when running the main headlights' high beams.
Very cool and the high beam turn signals are so close to the ground there's no glare to oncoming traffic along with no doubt about which way you're turning.