I can't find any blueprints or specs, but I found a handful of reference pictures, which might be useful:

http://www.radersrelics.com/showroom_2/1939_buick.html
http://www.classilink.com/10195.shtml
http://www.buick.com.au/
http://www.buickbombsight.org/39.html

I searched several plastic model kit sites, but couldn't find one. There is someone selling a model of one on eBay, here; if you want to spend the money, it might be useful as a reference.

Hope that's helpful,

Captain Jack