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I have to agree, there will be voltage drop at the bulb socket measured under a load, both if the wiring and switches are all new or 50 years old. Dad's '97 S10 benefited greatly by just adding a relay harness- there was a very noticeable increase in brightness even without changing out the headlights. Add in 30 more years of corrosion throughout the wiring and gunk building up on the contacts in the switch and there will be voltage drop.
The last two relay harnesses I've built used 12 guage wires straight off the battery into dual-87 relays, then more 12 gauge wire out to the headlights using ceramic sockets. I also ran 12 gauge ground wires straight from the headlight sockets back to the battery.
A good source for relays and sockets is Del City. They sell everything needed except the headlight sockets.
