Relays, H4 conversions
If you have gazillion-year-old wiring, you almost certainly will notice a difference in headlight output if you run relays. The problem isn't necessarily one purely of wire gauge or length. After 30 years or so (not sure of the age of your truck, but mine's celebrating its 36th birthday- 1970 K5) the wiring corrodes, the terminals get gunked up, and the switches especially become resistors. Take a DMM and check your voltage at the battery. Then check your voltage at the headlight connectors. I would be surprised if you see much more than 9-10V at the headlights given an engine-on battery voltage of 11-13V. The advantage of running relays is that you are no longer routing the voltage for the lamps all the way through the electrical system. The power to run them will be drawn straight from the battery (mine was basically a few feet of 12ga wire with an inline fuse in each circuit), while the long, corroded, worn-out circuit through your switches will only need to bear enough current to actuate a relay, something that even a prehistoric headlight circuit can easily do.
I rewired my old truck (67 K10) with a relay circuit for the headlights before installing Hella's H4 conversion lamps (similar to the LMC Euro Style lamps in concept). I also replaced the headlight connectors at the same time- no sense in running new wires to old, corroded connectors. Whole project cost me about $20-$25, and the difference in lamp output was huge. I also felt a lot more confident installing the high-output Hellas (which themselves made an even bigger improvement in light output) because I knew I wasn't running too much current through wiring older than myself.