In the early 2000's (I forget the year), we had bought an antique camper in Canada, so I went through the same questions back then with my Customs Broker. She looked into it for me and found that a vehicle over a certain age was exempt from DOT and EPA requirements, while if it were newer than 25 years (maybe it was 20?) and I were importing it, it'd have to comply with current EPA and DOT regs regardless of model year.
I just drove the beast over the border with the original Canadian plates on it and no fucks were given by anyone.
That camper was part of a long story...we picked it up at the compound of a Canadian-based cult that worshipped extraterrestrials and group swinger sex. Not making that up!