I just imported the '74 'cuda from Canada. As long as the car is older than 25 years old, it's a breeze since its exempt from EPA and DOT regs. If you had a non-American or Canadian built car, there would be more hoops to jump through.
The best thing to do is to take it across the border yourself. That way, CBP just fills out a Form 7501. It took about 5 minutes and there was no charge. Make sure you have a Bill of Sale. I also bought a 5-day Canadian permit/registration for 38 CAD, though not sure that it was required.
I'm sure each State has its requirements. The Texas stuff was listed out on the DMV website and was pretty straight forward, but have patience since these people don't import cars everyday.
I understand that going the other way (US to Canada) is more difficult.
Good Luck!