If you take a look at gun trucks early in the Iraq war, you'll notice very little standardization. All kinds of trucks from HUMVEEs to HEMTs were armed variously and armored with whatever was at hand ("hillbilly armor" they called it). Only later did you start seeing purpose-made armor kits, turrets, and that sort of thing.
I think that, in the Twilight War, by the time the need for gun trucks became apparent, no one really had the manufacturing capacity to create "official" armor and/or armaments packages. Therefore, gun trucks would be created ad-hoc, with whatever armor, weapons, etc. that the creators happened to have on hand.
We had quite a discussion on gun trucks a while back, James. There may be some useful info and pics here, if you decide to make any revisions. Here's the link.
http://forum.juhlin.com/showthread.p...ight=guntrucks