|You deal with the Ins Co. on the value and they create the premium accordingly. You'll pay more for a higher agreed value.
It's what I did with Hagerty with my '69 roadster... I had a professional appraisal and recorded modification expenses. Car is agreed value insured for 20K.
From Hagerty: You tell your insurer what your car is worth (with proper documentation) and it is insured for that amount. ... With Agreed Value coverage, the insurance company will guarantee that they will pay this agreed-upon value in the event of a covered total loss.
Hagerty Stated vs Agreed Value