My current car is a production car so it has a standard blue book value that I assume my town is using to determine the personal property tax on it.
How is a car valued if it is a kit car or custom car built by the owner?
The Vehicle Excise tax in Massachusetts is administered and collected by the local municipalities, but is based in procedures set out by state law and the state department of transportation.
The valuation used for "normal" cars is not actually based on the current condition or worth of the vehicle, but a percentage of the list price of the vehicle when new, with the percentage decreasing each year until the vehicle is 5 years old. From the Massachusetts Department of Revenue Frequently Asked Questions on the Motor Vehicle Excise:
The excise is based on the value of the motor vehicle as determined by the Commissioner of Revenue upon certain percentages of the manufacturer's list price in the year of manufacture. The excise valuation is not based on the actual purchase price or "book value" of the vehicle. The percentages set forth in the statutory depreciation schedule that are applied to manufacturer's list price are as follows:
In the year preceding the designated year of manufacture 50%
In the year of manufacture 90%
In the second year 60%
In the third year 40%
In the fourth year 25%
In the fifth and succeeding years 10%
Getting information for custom cars was a bit more challenging to find. There doesn't seem to be any provision for them in the relevant law, Chapter 60A, that I could find. The end of the Secretary of the Commonwealth's Motor Vehicle Excise Information references a broken link to a "Motor Vehicle & Trailer Excise Manual", which I found a copy of on the City of Boston's site with a search engine. It includes this information:
While most vehicles are valued electronically, valuation programs are not available for all motor vehicles or motor vehicle components. … For those motor vehicles which cannot be valued using the available tapes, the Registry of Motor Vehicles annually ascertains valuations (default valuations) and mails notice of these valuations to all board of assessors in the fall of every year. Assessors possess authority to abate a Registry default value in any case in which they believe it is excessive. Assessors may also obtain the manufacturer’s list price of any particular motor vehicle by either (a) calling an automobile dealer who regularly sells that brand of vehicle, (b) requesting the vehicle’s registrant to provide the window pricing sticker of the car when new or (c) calling the manufacturer’s customer service number.
Like with most valuation-based taxes, the government will take its best guess as to the value, and then it's up to you to file an abatement if you think the valuation is overstated. I would recommend that you contact your local Board of Assessors to see if they have any information on how they've valued similar vehicles in the past, or if they can give you any further information on the sources they use to value "unusual" cases like the ones you're interested in. (Answering questions like this is literally the job that they're being paid to do, so I think they'd make a great resource.)