Wiring harness dilema

[Lloyd C.]

I'm in the process of switching the dash in my son's 66 C-10 over to a gauge cluster with gauges. The truck originally had only warning lights. I have a brand new under dash wiring harness that came with the truck. The dash cluster harness has a part #5500 and is labeled "64-66 C-10 with out gauges" while the main under dash harness has no part number and I have no idea who the supplier was. My question is: Do I need to change out the entire under dash wiring harness or only the smaller, and less expensive, dash cluster harness?