I like the idea of using LEDs as daytime lights
What LED bulb fittings and power are recommended to substitute for the standard and probably rusted-in bulbs in the side lights mounted on the wings?

Careful here. From the early 70s to the present, cars are required to have E marked bulbs and fittings. It follows on that none of the aftermarket LED fitments are legal. It's a very complex area.

Situation is different with older cars as the use of E marked bits is not required so my three wheeler does use some LEDs legally.

For your usage, what you need to do is to smear new sidelight bulbs with a thin layer of conductive grease. A tube will last a lifetime, my tube is still 2/3rds full and it was new in about 1988.