Not sure if this is the best way to go about it, but if it were me I'd probably actually ask them "Since this was an enthusiast vehicle for me and I did all my own work/maintenance to keep it in pristine condition, what form of mileage documentation would be satisfactory?" I mean, at the end of the day, it was just you writing down a bunch of numbers on a piece of paper, how is that valid proof to them in the first place? Or are they just trying to be a pain in the ass? I feel like that question would sniff that out.
I'm not sure I would bring your past ad into the picture unless you absolutely have to. Especially considering the price difference. The vehicle's value could have very well gone up due to it being rare and desirable, but without comparison ads to demonstrate that I think it would only hurt your case.