For a factory-style harness, I went with AAW Factory Fit and bought everything for the car when they had a 15% off sale. Car's not finished so I haven't run everything yet, but it's the closest I could find to what remained of the original wiring. Some have noted minor inaccuracies for Judged events, such as incorrect color connectors in one area, but nothing major IMO. Also check out Lectric Limited, no personal experience of them but know some owners who highly rate their harnesses.
If you don't need an original-style harness, then AAW offer the Classic Update kit that uses modern blade fuses, and Painless also offers a similar product. These are good if you want to upgrade to modern accessories.
FWIW, I watched a recent episode of Finnegan's Garage on YouTube where he completely rewired a '69 Firebird with a Painless update kit.