I've done about 3 wiring jobs since i finished my z about 5 years ago, so i have a little bit on knowledge on the z.
I to used a hotrod universal 21 circuit wiring harness very similar to the "painless" harness except it was about half the price. I'm going from memory here, but the steering collum wiring from the z was the most difficult part. The stock light switch actually grounds the circuit to the headlights, there is a constant POSITIVE 12 volts to one side the head lights. I installed 2 relays to take the load off the switch.
Other than that everything else was just following the wiring's location written on the wires themselves. My first full wiring job was my Z and that took me 2 weeks of on and off time.The last job I did was for a 69 chevelle that took me 2 full days. I know it looks intimidating , but just separate the workload down to smaller sections. Work one circuit at a time and you'll get through it. I hope that helps!!!