The reason you want to install the relays, is the Headlight switch contacts aren’t designed to carry the type of amperage that Halogen bulbs will pull.
There are several places to get good quality electrical parts and hardware. The terminals for the headlights are 5/16" and are available at Ron Francis
http://www.parts123.com/parts123/yb....rta~partsort~2 P/N AM-8 you get 8 for $7.95. Or you can get 50 if you think you need that many. This way you can just buy bulk wire and make your own harness. The other ends are 1/4" and can be had at MAD Electric
http://www.madelectrical.com/catalog/flat-blade.shtml P/N 10-56F10 you get 10 for $2.99 or again 50 if need be. By the way the 5/16" terminals aren’t readily available over the counter. I tried every where to find them and finally had to order them from Ron Francis. He was the only one I've found that carries them. The 1/4" one can be had at NAPA and are easily found on the internet, they are used everywhere else I've looked on my truck for the female terminal end, and I've replaced a lot of terminals. Oh if your headlight sockets are burnt up or the terminals don't lock in place because the sockets worn out just go to any auto parts store and look for one. Auto Zone, NAPA, they all carry it. Here's the link to Auto Zones web page
http://www1.autozone.com/servlet/UiB...ction=loadPage P/N 85810 for $2.99 each.
You can either buy the fancy wire from MAD or just go to your local Auto zone or NAPA and pick up some bulk wire. You'll need Brown, Green and Red and some Black. Then just make up your own harness. Relays, I really likes the ones at Parts Express
http://www.partsexpress.com/pe/showd...number=339-075 P/N 339075 for the socket and 330-077 for the relays you'll need 2 of each. Also fuse holders
http://www.partsexpress.com/pe/showd...number=070-675.
The H4 Lens can be had a JC Witless
http://www.jcwhitney.com/autoparts/I...D:100000165791 P/N ZX139317U $17.99 They also have replacement bulbs all different wattages.
It took me about 3 hrs to solder and make up the harness and about 2 hrs to install that was using all the parts I listed and bulk wiring. I also used from MAD
http://www.madelectrical.com/catalog/cn-1.shtml P/N CN-1 $6.99 each. Get several of these they work great under the dash on the fire wall etc.
The H3 bulbs are for single element lights i.e. separate hi/lo beam like the GMC's have. The H4 and the sealed beams that came with the trucks whether it’s a normal sealed beam or a halogen sealed beam use the same socket so they are interchangeable. When you start getting into the 9000 series they are all of the newer "weather resistant" type connectors from the later 80's and up. You can get and install a harness with the 9000 type socket and just run that type bulb in the H4 lens assy if you wanted. But I don't know why you'd want to.
It's easier just to use your original socket take a small screw driver and insert it into the end the that the blade from the bulb slides into the terminal to release the small clip that holds the terminal in place. Pull the wire and terminal out, and then replace it with a new one. This is if you’re building your own or the wires on your kit are too long, or you get the wrong kit and just want to try and use it with new terminal ends. Or you can splice the wire but that looks cheesy to me. If your gonna do it then do it right. Electrical work isn't hard it just takes a little practice and patience.
The H6024 or 6014 bulbs are sealed beams. In other words the bulb and lens are one assembly. Whereas the H4 lens and its corresponding H4 bulb listed under dozen of P/N depending on what wattage and manufacturer you choose is separate. The H4 designator stands for a design type. The lens is installed like your regular sealed beam headlight. On our trucks you have to trim out the headlight bucket for the back of the lens to fit. Then you just have to replace the bulb itself when it blows out. The bulb comes in a multitude of different wattages. In the Wiring Diagram below there needs to be a 30 amp fuse or Circuit breaker when it says “Direct to battery terminal”. This will protect your truck from a short that could possibly burn it to the ground.
I’ve done this mod on at least 6 different early 50’s to 90’s model cars and trucks, They ALL improved the performance of the headlight noticably.
http://67-72chevytrucks.com/vboard/a...1&d=1160589844