I scarfed them off of some equipment in work headed for the scrap pile, but they are available commercially :

http://www.carrlane.com/Catalog/index.cf...A3C3B285350415F

Although mine isn't quite finished in the picture, I made it (if I remember) 36" to the center of the rails. Thought being, it splits the difference of how the frame rails vary in width on the car. I then made the uprights mimic the frame dimensions in the above post.

Jeff @ 521Restorations has a nice write up on his as well.
http://www.521restorations.com/index.pl?page=framejig