I just did this on my 86 K5.
https://www.gmsquarebody.com/threads/instrument-cluster-swap-from-idiot-lights-to-full-gauges-k5-k10-k20-k30-c10-c20-c30.45632/#post-966363
You need to get another complete gauge cluster. The idiot light cluster isn’t setup for the gauges and you would have to do...