I subscribe to Classic Trucks. They had an article 2-4 months ago that had every single measurement for trucks from the 30's to the 90's related to seat width, height, and spacing off the floor. It was the ONE magazine my wife threw out, and of-course I've got the same problem on a '55 (First-Series) Chevy I'm rodding.
Check out http://www.classictrucksweb.com/
and see if they can get you a reprint (it was either June, July, or August mags... I have all the other ones and it's not in them).
Oh, and don't leave your copy lying on the table for your wife to clean.