Does anyone know what year Spike's DeSoto was?  I love that car, and I want to get the model-year right in my fic. :)

On the BtVS Trivia Guide there's no such information. It only says: "Spike drives a Desoto Sportsman, with blacked out windows. The license plate is a Californian one - HIA 873." Maybe you should show a picture of this car to some guy who's into cars, or maybe put it on a forum about motoryzation?

According to Ebay, where I found a "Johnny Lightning" replica of the DeSoto, it's a '59.

Wikipedia confirms this, as well.

 http://www.fangedfour.com/travelogue/id3.htm

Yeah, it's a '59. If you look up Desotos, most years differed from every other year, and it's easy to tell the difference. Desotos were once known for their creature comforts and advanced engineering.

I've got a book of American Classics if you want specifics. :)