I have been working for a custom truck body manufacturer for a week or so now and I have discovered a great source for scale drawings of modern trucks from all the big manufacturers. My company does what's called upfitting for chassis cab units. Do a google for upfitters or body builders for the make of truck you want and you should find resources for upfitters like us.

Here's Ford's upfitters site with direct links to their pdf book for body builders. It includes some good dimensional drawings of their light and medium duty trucks.

https://www.fleet.ford.com/truckbbas/index.htm

Here is Chevrolet's:

http://www.gmupfitter.com/wwedo/wwd.htm

Here is Dodge's:

http://www.dodge.com/bodybuilder/index.html

Pretty cool and usefull site for the Scratchbuilder