Based on my experience over the last 10-15 years -- "collector car" and "agreed value policies" go hand in hand. I'm not aware of anyone that will do an 'agreed value' policy on what is basically an older car in great shape. At some point it crosses a line and becomes a collectible. Sadly, the point at which that occurs is subjective. We may, and usually do, see it differently than the insurers.

Until then, your best bet is to keep track in detail of all you've done to preserve it AND have it professionally appraised regularly. Then should something untoward occur, you at least have some 'ammo' with which to argue it's value with the insurance company responsible.