I have been trying to answer that question myself about my '66 K20 and am having a difficult time getting a professional appraisal. I even contacted Hagerty insurance to ask for value range and none was provided. Online, their listing only goes back to '67.
The only real life number I can share is what I paid for mine in Feb. of this year in running, driveable condition (pics and description in this forum), and that was $3200.00 that included 2 pickup truck loads of spare parts for that truck.
