It sounds as if you are doing a frame off restoration on the car. If that is the case, I would absolutely go with new wiring throughout. It means more work and will most definitely cost you more during the resto; but, why take a chance of bad wiring causing you problems (like a *gulp* fire) later on?

Check out Ron Francis (as Streets already suggested) Painless also makes harnesses as well and there are others. Anyway, you can get them pre-fabbed for your vehicle, (Check out Summit that's where I got the one for my Firebird) or you can buy a 12 or 18 circuit harness and cut it to fit. The wires will most likely (all of them that I have seen have been anyway) be pre-stamped with where to connect them. While this doesn't take all the work out of wiring a vehicle it absolutely helps.

Also, HWORRELL mentioned only using soldered joints and heat shrink and not crimp connectors, that is an opinion, and it is one that I agree with 1,000%. Crimp connectors are faster, but since when does faster necessarily mean higher quality? I would also suggest using what are known as Western Union splices, and then soldering and covering with heat shrink. If you don't know what I mean by Western Union splices, it is a way to wrap the wires together so that it essentially won't come un-done when you pull on it...before soldering it. Here is a link to a site that shows how to do it.

http://www.tpub.com/content/construc.../14027_122.htm

One final thought on the subject, I've always felt that if the wires are routed correctly, it is one of those areas that will contribute to looking like your vehicle was done with outstanding craftsmanship, if routed haphazardly, it will look like shit, no matter how good the paint is or anything else. Take the time to figure out exactly how you want the wires to be run (especially under the hood; but, elsewhere as well) before you actually cut or connect anything...ie, do you want them exposed and easy to see, or do you want them hidden from view as much as possible? I am always amazed at how well wiring can be done to make an unfinished car in primer and still needing quite a bit of body-work look awesome, and how one that is actually "finished" with wires running all over the place makes it look absolutely horrible. Also, when it comes to running your speaker wires from the stereo, run them as far away from the engine compartment as possible. This will help eliminate engine whine in your speakers when the radio is on.

Lots of luck,
Firechicken