I'm not familiar with Canada to US importation, but I suspect it should not be as big a deal as going the other way. Anything over 25 years old is eligible, without restrictions from what I understand. Also, there's a high probability this is a re-importation i.e. a US car that went to Canada so if you can substantiate that it should be much easier to bring back 'home'.
If you are seriously considering the car, then I would;
a) Call a US customs border agent at the crossing point you expect to use and get some questions answered as to what is required. Take notes and names of who you speak to so you have reference when you actually arrive.
b) There are customs agents who contract to do all the work for you...for a fee. Call one and get some of idea of cost and what you need to provide in terms of paperwork etc.
c) I'm sure numbers of classic cars come back in to the US annually (although there has been a mass exodus to Canada of recent years with the US$ to CAD$ exchange rate). In terems fo panetra's Daniel #9191 brought in his GT5 I believe, so it may be worth dropping him a PM, to find out how he did it.