There are some Official (?) drawings of various GMC models on William Marshall's Webshots Album here
http://news.webshots.com/album/181012271UcuRkt when viewing 12 to a page they are on pages 17 & 18. These have helped me get a clearer understanding of the various models discussed here.
Thanks to Hanno who originally posted the link elswhere [in the Armour section], some interesting stuff in there.
Noel