I just went through this as well. I have been with farmers for years, and their claim service really pays off. With the changes in colorados insurance laws, (not true no fault any longer) and the fact that you still have the option to use your carrier in an accident where it is not your fault, I wouldn't mess with the stated value. The cost was not worth it in my case.
As for vintage, I have had those policies, and they are mainly designed for limited use vehicles, and comprehensive damage. They are affordable for motorcycles, and stock old vehicles, but that's about it. As for it being totalled in an accident, good luck. That really just comes from trying to replace something that there are no more of, not the policy.
My lesson I have learned is that pictures really are worth a thousand words. They have proved my case every time. Here's why. If you have pictures of the accessories on the vehicle, and it matches the pictures the adjuster takes, who can argue? I keep a photo album on my pc, and make sure that I have pictures of the vin included, showing the paint with the tag. Meaning not just a close up of the tag.