Go prepared to meet the insurance agent -- it's going to be a battle.

1) Go to the web (lots of "used cars for sale" sites such as Edmunds, etc.), and printout (including URLs) ads for similar cars;
2) Go the eBay, and get "closed" or ended auction prices (not the ones still open, since prices jump at the close) -- print them out, too;
3) Go get a copy of Hemmings, a hard-copy publication on newstands, and pick out the ads;...

then compile all that you find, the more the better, and put these sheets of paper in a binder, like a school report. I'm serious -- you want the guy to know that you mean business, that you're going to provide evidence of the real market value of your pristine car, and that your car is not adequately reflected in some ordinary "average" used car price for an over thirty year old car!

...and for those ads with high prices, carefully make a detailed list, writing specifically why your car is even better than those being advertised, such as "lower mileage", better condition (pictures help), recently fixed or upgraded, etc.

The only way you will win is with hard facts, facts that he can take back to his supervisor. The fact that you love your car means nothing, but it's market value means everything.

Good luck.