Measure the wiring for voltage drop. It will tell the tale. Run engine at high idle - say 2000 or 2500 RPM. Turn headlights on. Place one probe of voltmeter at center of negative battery post, the other at the negative terminal on headlight. Should not exceed more than 0.2 volts or so, maybe 0.4 volts total - both sides, positive and negative circuits.

Typically battery and ground cables are junk after a couple decades and should all be replaced. Grind down boss connections to bright shiny metal and tighten securely. Charging systems, generator or alternator do not like high resistance in the circuit. Headlights have a "strike" or minimum voltage they require before they will operate properly, brightness drops off quick at anything below this figure.

Also, if the headlights are dim, then the ignition and accessories will be "dim" as well. The charging system "sees" excessive resistance as a battery that needs charging constantly. These old trucks will have corrosion problems after these many years and it takes a little effort to get them back to OEM levels. Restored trucks often have similar issues, though with thick paint performing interference duties.

Shiny connections = Happy electrons

 

Yes, that bulb will fit your truck. Please also follow the advice about clean connections and good battery cables.