[Accelo]

"agreed upon value and do not require an appraisal"
But they do require pictures. So one can't purchase a junker and insure it for 50K.
I'm sure someone has already tried that.
I would look at the selling price of similar trucks. They vary a lot in value depending on options and the truck style. Most appraisers seem to have a large amount of latitude on valuation. The trick is to cover the truck, if a replacement is required, and not over pay for the insurance because a high valuation. Because of the sentimental value I bet it is keep inside and driven like an old man. The chances you will need the insurance is low.
I believe the $40K policy is close to what i would recommend knowing nothing else about the truck.