Step 1 is to determine if truck is actually overheating. You do that with a temperature gauge. I assume you have no water leaks. Be sure you don't. You did not say how you determined it overheated. On some radiators, you need to just fill above the coils and leave some expansion room or it overflows a little but that is not overheating. That is why a temperature gauge is needed to see if motor exceeds about 200-210 deg F. 220 F is shut down time.
The wrong cap can let water "boil"/evaporate out while driving. Other things can cause "overheating" besides inefficiency. You can have overheating due to fan not blowing/sucking enough/too far from Rad, motor overheats and pressure lets water out cap or...... you can have loss of water, blocked radiator, radiator too small, collapsing hose, bad water pump, loose fan belt, bad thermostat......... and then overheating.
Step 2 is be sure you have a new thermostat. Use the recommended one for the year of motor.
Hard to say what radiator cap to use. If radiator is original, get a new aluminum one. An old rad and a newer V8 usually does not work.
Your fan is WAY TOO FAR away from radiator. Many shrouds available. Some can be trimmed/modified to mount/fit. As Jerry says, fan should be half in/half out of shroud.
Be sure fan centerline is approximately at radiator centerline in the Y axis.
These updates/new parts/shroud should do the trick with no electric fans needed in your area of the world.
Yes a donor fan from some other vehicle may fit, but finding which one is difficult.
