Some cars are easy and some are hard. If the car is more than 25 years old and unchanged from its original configuration (IE still has the same motor type it had when new) then this is an easy import.
If the same car was sold in the US or you can find a letter from the manufacture that says it meets all the US requirements at that time - easy. A friend recently imported his parents Dodge something or other that they gave him. As it was the same car sold on both sides of the boarder he said it took about 15 min to cross the boarder.
Everything else is much more complicated to import.
See this web page for some tips
http://www.clubgwagen.com/classifieds.php
In the long run reality always wins.