If you haven't purchased a harness yet do yourself a favor and check out the American AutoWire offerings. Their kits include everything you need to wire the car, right down to switches and bulb sockets if you need them.
American Autowire: Wiring Harnesses, Accessories, & Parts for GM Restoration, Modified Restoration & Custom Street Rod
There instructions are very good also. Summit and Jegs both carry their kits as well as Speedway, I believe.
Mark