Lucas style headlamps units for Brit cars will fit straight into your buckets. I have classic (i.e. '60s) Mini H4 headlamps in my '50 truck -these have bulbholders for parking lamps built in too. I use the original park lights as turn signals with Europe legal orange bulbs installed.
Don't suppose there are many classic Minis in the US, but MGA, MGB and Triumph TR3,4,5 and 6 all use the same size headlight as the Mini, so shouldn't be too hard to find with H4 bulbholders where you are.
Don't know anbot the Bonneville lamps you've been looking at, but most Lucas motorcycle headlamps were made for "vertical dip" so won't light up the edge of the road when you switch from main beam -certainly my '63 Triumph 500 had this setup fitted.
Stuart.
