Is a 1983 C10 wiring harness interchangable with 1985 C10?

No, a 1983 C10 wiring harness is not directly interchangeable with a 1985 C10 wiring harness. While they share similarities, there were enough changes in the electrical systems between those years (particularly with regards to emission controls and added features) that the harnesses are not plug-and-play compatible. You'll find differences in connector types, wire gauges, and the overall routing. Attempting to use the wrong harness will lead to significant electrical problems.