Must be 25 for any kind of collector car insurance
Just show them receipts. They'll insure the car for up to what you can prove. I'd just show them the major ones. Don't want to make it too expensive, but at the same time it's nice to cover 75-90% of the car.
Try Allstate, State Farm or Nationwide. They have Stated Value policy. Not as good as Agreed Value, but if your daily driver is with them, they'll probably take the collector car policy too. Keep in mind this type of policy is not for a daily driver and it requires the car be parted in a secured building (garage, storage unit, etc.), and the age thing may or may not be a problem.