"That's a very generous starting point."
That is also a generous way of saying something to you in a very nice way. The insurance company is not in the business of selling your truck. They are in the business of making money through policy premiums. My advice would be to drive that sucker off a cliff as soon as possible and collect the 10 G's!
Less than obvious modifications or not, there is no conceivable way that you can get someone to write you a (cashable) check for $10,000.00 in this, or any forseen economy.
An agreed value policy does not mean that your vehicle has been appraised by the ins. co. and that is now the market value of the truck. It simply means that they will charge you premiums based on 10 grand. Nothing more. I agree with GMONIZ' value assessment. Old trucks are a hobby not an investment.