I used the harness because I hate wiring. I don't think you need the harness, but it does make installation easier. I have the factory wiring, so it made it much easier for me. I still had to splice into the factory wiring even with the harness. But it does have its advantages. Instead of having separate grounds for all the gauges, I think you only have two. Also if you need to remove the dash, all you have to do is unscrew the two grounds and two connectors. Good luck.
1969 Camaro 350, Trickflow heads, ST-10 4spd, Hotchkis suspension, Baer SS brake kit, moser 12bolt