Hey Doug:
As a former assessor of real and personal property - you are fighting an uphill battle. The juice is not worth the squeeze, as we say. Your assessor/ tax collector only knows how to follow convention, and since your car is probably one of several thousand in the community they hate to set a deviating precedent for you.
That said, you are right- and this is purely an argument about value. The standard they use to approximate ad valorem assessment (which is the statutory requirement) is NADA but that doesn't mean that it is accurate. Outside of taking it to your appeals court which will cost you way more than you will save, you could get a licensed appraiser to issue an opinion of value or swamp them with data on parts cars that are for sale currently. There are a few parts cars out there which are for sale for $1,500 -$2,500 and if you had some pictures of your car at the time of purchase you could draw the analogy. Lastly, yesterday I agreed to buy a 65 not running or rolling but in very good condition for $5K. You could use that to argue prices are higher than 10 years ago when you bought your car so the $3,500 is more accurate than $10K.
If I can help you give me a shout.
Brad