Back in the day 12k a year was a pretty fair estimate for a privately owned vehicle. Based on what I am driving for work now, 2007 Tahoe with 249k miles, it would be anybodies guess for a vehicle with a commercial background. BTW the Tahoe looks and runs like a new car except for a lifter tick that won't go away.
So your truck is 58 years old, out of service for 30, 28x12k....could easily be 250k for a private vehicle.
