You have several options, depending on how ambitious you are.
1. Make your own from pieces salvaged from other trucks - I do not recommend this. Time consuming and you still get 48 year old wiring.
2. Make your own with new wire. I have no idea where you could get the correct connection blocks (new).
3. Buy a pre-made kit from Painless or American Autowire or Rebel or . . . . . Each kit has new wire and connectors and is ready for upgrades to include A/C and powered stuff that you might need one day and saves a ton of time. But not cheap.
4. Buy all new wiring from M&H Wiring
. That's who Tony at 4-5-6 Chevy Trucks recommends if you want original style and factory correct NEW wiring harnesses.