The AFV Club quad .50 guntruck "Nancy" is inaccurate in the sense that the gun turret is depicted as a pedestal mounted to the bottom of the truck bed, when in actuality the gun was a separate trailer temporarily attached to the truck bed that could be removed and installed as needed.

Were they all that style, or did some of them have the fixed pedestal as in the AFV Club kit?

I never saw any without the trailer. I have seen them on a stand alone set up on the perimeter and lashed down in the back of both the 2 1/2 ton and 5 ton trucks. There as two that I have seen that were on the back of a tandem trailer that was pulled by a 2 1/2 ton truck that also had a quad in it's cargo bed. I guess you could bolt one down to the cargo box floor though. With the way we went through trucks it is a lot easier for the crews to load and unload the guns if left on the trailer.

Here is a link to some quads in Nam
http://www.landscaper.net/theguns.htm#Gun%20Trucks%20-%20Quad%2050's%20and%20Mini-guns

I suspected that trailers would have been used in all cases, if for no other reason than it would make the quad gun easier to remove for repair and maintenance.

Now all I need are directions on how to make the trailer, either the stand-alone or lashed-down versions.

Here is some info on it from Jim "Guntruck" Lewis' Nancy build.

http://www.guntruck.com/ReviewM35A1Nancy.html
http://www.guntruck.com/ACENancypage1.html

His is a great model and an accurate rendition, with the trailer added.

Also, the trailer comes in the new AFV Club M548 Guntruck model.

Some interesting Vietnam-era Quad .50 pictures can be found here :
Quad.50s
or on the website of the National Dusters, Quads & Searchlights Association
HTH

Frenchy