Gazz, if you decide to add light relays (a good idea), and want to be "period correct," there are usually several listings on eBay for NOS Delco PN 116789, sometimes for a reasonable price. These are 6V units, and were used as a dealer installed option through at least '53 for cars, specifically to aid with customer complaints about dim headlights. Just be aware, some sellers don't know what they have, and it's not uncommon to find a used unit that was put back in a box, so the seller thinks NOS (or hopes the buyer will think so) when in fact the part is not. Or, simply add a modern relay, or 12V unit if the truck is converted to 12V.
Here is the installation sheet dated 4-10-40 for the Delco part, which may be helpful even if you decide on a new relay (or two).
