First, you need to tel us exactly what you are buying:
Who/How manufactured (like, Intermeccanica, Vintage Speedsters, Home-built CMC, etc.)
If it is a CMC, Vintage or JPS and based on a VW pan, you can, in Massachusetts, register it as an original volkswagen for the year of it's manufacture (for instance, mine is registered as a white, 1969 VW convertible, and the Mass DMV is OK with that.)
If it is a tube-frame based car (some IM's, Beck and SAS) then you need to register it as a custom automobile and it can get a bit more complicated, so look here:
http://www.dmv.org/ma-massachusetts/custom-built-cars.php#Current-Procedures
Massachusetts adopted, last year, the SEMA based bill for registering and insuring custom built cars such that you register it as the year it represents, i.e., a car that looks like a 1957 Porsche can be registered as a 1957 Porsche and is emissions exempt. I'm not sure if the law has taken effect yet or not (I don't live up there any more). That all sounds great, but not a lot of people working at the DMV (and, especially, local inspection stations) know what's going on with the law so it's best to call the DMV, Tell them what you're trying to do, get a competent person to help you with the right steps and have them call the state inspection station where you'll have your safety inspection done so they won't screw it up (see Al's post above).
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