I live in Mass also and when I was building my 29 Ford Roadster, I questioned the Revenue department about the fact that I was paying sales tax on a lot of the items I was purchasing to build the car. He told me, ( don't remember his name) that a lot of the receipt that I paid sales tax on could be used to offset the final tax on the vehicle. He mentioned the motor if I purchased one and most of the chassis components. I don't remember what else was usable. I sold the car before I finished it.
I had a title for the Flatheadand registered it as a 1941 UL. And it sure ain't anything like they built back in 41.
Also stay away from New Bedford (I don't live there but work there) when getting a Mass. tag. The guy down there will not accept any receipts unless they are from a dealer. He will not accept personal sales even with a receipt. I am wondering why no one has gone over his head.