There are several tech tips on this that are helpful, here's one:
http://www.stovebolt.com/techtips/12_volt_conversion.html I'm in the middle of the same thing and suggest you upgrade to a new wiring harness with modern fuse panel and relays. There are some vendors that make a replacement wireharness for your truck that are 'bolt-in replacements', like jim carter, and may also sell a replacement harness with fuses for a few bucks more. These tend to be expensive, when I looked they were in the $400 range.
You can also buy a universal harness which comes with wires that have their purpose printed every six inches from someone like Painless, EZ Wiring, Its a Snap. Those ones are cut to fit and often include the relays for headlights, a/c, horn, and flasher for turn signals. If you go that route you'll have to decide how many circuits you want- I got one with 18 circuits on e-bay for around $125 and now I think I probably won't ever need that many. Yesterday I saw smaller ones in the Summit catelog for much less than I paid ($50-$60)- I think 6 or 8 circuits is plenty for a truck like this and next time will count them up and only buy what I need. Some say get a bigger fuse panel 'cause maybe someday you'll add power windows, electric seats, etc...' Some panels you can buy in blocks- 4 fuses now and then a new panel of 4 that clips to the first.
You will also need to change all the bulbs to 12V and put a voltage reducer in place for a few things-ie gas guage or 6v accessories like the heater blower motor or radio. When you upgrade the headlights to 12V for almost the same money you can upgrade to H-4 lenses and bulbs instead and get more light.
If you keep the original 6 volt ignition system rather than converting to 12v by using 12v coil/points/condensor, or upgrading to HEI, you'll need a voltage drop (ignition resistor)for the coil to get it from 12 to 6, or you could maybe get one of those VW coils that are 12 v in and 6 v out.
You'll need an 12V generator or 12V altenator and 12v battery- For the battery, take the old one to the parts store and get one with the same dimensions. For the alternator, I'd go with a 3-wire say from a 74 monte carlo- You can get one with a lifetime warranty from your FLAPS for $50 or so. If you've got lots of electrical accessories you may want a higher amperage alternator. You can mount the alternator where the generator was with an aftermarket bracket or, as a tech tip suggests, with some thread=all and bolts using the generator mount. If you've got a welder you can weld a tab onto the generator mount or make your own bracket. If you go with an alternator you can get rid of your voltage regulator. If you go with the alternator you'll need a DA plug- The three wire alternator has two spade terminals (no1 and no2) and a 'bat' terminal on the back and there was along post here about a week ago on how to hook it up.
When you are rewiring your dash lights you could use new 'pigtails'- Those are the discs with metal contacts in the middle that go in the bottom of the bulb socket. ECHLIN part number LS-6227 (at napa). In the same line of parts they have two contact pigtails to upgrade your parklamps and brakelights to turnsignals, if you havn't done that.
Relays for the headlights, horn, electric fan/fuel pump or A/C, if you've got them. In the case of the headlights it'll reduce the load on the switch and possibly increase the juice to the bulbs a little, making them brighter.
Alan Horvath has some great info on his website about this upgrade and relays.
