I have read several articles about 12 volt conversion. I will need to do this to add a 7 way plug to utilize my trailer's electric brakes. Those of you that have converted, did you buy a conversion "kit" or did you do better buying the individual items and make your own "kit".

What year?
Up to 50’s can be as simple as new light bulbs, an alternator, and resistor each for coil and fuel gauge.
Later might be more complicated but I doubt by much, I’m just not familiar with the newer trucks.

You pretty well nailed it Grigg. Anything mid 50's on up is all ready 12 volts. He will probably use a 12 volt radio. Put a resister on the heater circuit or a 12 volt blower motor. I have run 6 volt heaters on 12 volts.

I forgot about the heater. New 12 v motors are generally available for about 30-$40 at local auto parts store.
Search in electrical forum for details on that.

In my experience the 6V starter will be just fine and dandy, and crank with authority on 12V
https://www.stovebolt.com/ubbthread...7279/Easy/cheap_heater_blower_motor.html

I converted my 51 to 12 volts. I used a 12 volt generator and regulator, resistors for the heater motor and another type for the the gas gauge. I used a 12 volt coil that does not require a resistor, as marked on the side of it. The heater resistor is 1 ohm 50 watts and the gas gauge resistor is 30 ohms 2 watt.

I used a reflector type headlight that uses an H4 bulb. They are very bright with a modern type pattern. They look almost identical to a sealed beam unit.