For the current project, I am mounting them on the back of the headlight buckets, where they will be mostly out of sight and yet easily accessible (by pulling the bucket). The main harness will remain intact and original, with the only visible mods being a new Lucas fuse block near the original one, and two wires from the new fuse block to the relays. My theory is that the headlight pigtails will also look fairly stock, except for the extra wire.
But I haven't gotten very far with it; got distracted when I realized that the original brake light switch only comes on when I'm about to lock up the brakes. In between rain drops, I've added a bracket to mount a mechanical switch so it rides on the pedal. Here again I wanted to hide it, so it's under the dash this time (TR3A had it mounted on the pedal box under the hood, where there is a very convenient original hole).
PS, I also discovered that my right headlight is losing over 1/2 volt inside the original headlight connector. Both sides will be getting new connectors along with the relays.