Glenn - so far I've only built one quad 50 and it was by Dragon. The build report can be seen here:
http://vietnam.net.pl/M55mod1en.htm
Like Tcoat wrote, the barrels would have to be practically parallel. Now that AFV Club quad is somewhat old and has a lot of misunderstandings, but is still a step up from the gun truck "Nancy", because you get a resin trailer chassis that is correct for all quad 50s in Vietnam. Some parts in the kit, however come from the half track M16 version - you have to watch for those when building this kit. The power charger (small gas engine/generator) would have to be replaced with something newer here.
Rob - That's interesting! So what do you reckon, how many hours do you have in one of these babies? Like I wrote above, this kit has a substantial commonality with "Nancy" gun truck, but improves on it.
Tcoat - maybe I'll get to building a gun truck on wheels some day too...
John - a plain M548 seems to be a pretty common vehicle, but it has some wild variants, too, and those can be rare.
Thanks a lot for your comments and have a nice day!
Paweł