You'll want to look at the Ford 2006 F-Series Source Book for complete
vehicles specs. The dealers have them in big binders. My dealer
has several out in the show room and, of course, their fleet sales people
have them. There's an online version at the Ford commercial trucks
website, but you need to be dealer employee or a qualified fleet
buyer with an ID and password to access it.
The Source Book has very detailed information on the vehicle features,
options, power train specs, dimensions, and especially the curb, cargo,
payload and gross weight specs.
In the past I was able to make copies of the pertinent Source Book
pages so that I could study the specs at home in my leisure before
putting my order together.