Hello Glenn!
Did I ever show you this thing I've built?:
I even had not one, not two but three build threads here on the forum - I'll post a link if you want to. They are all summarized on my web page here: http://www.vietnam.net.pl/M55moden.htm
But you are buildind something else... Here's my idea for you: Get the Dragon M55 Quad trailer and use most of it in your model. Now the biggest problem with the AFV Club model is the wrong base for the quad - the model shows a M16 (Half track) - style base for the quad, in Vietnam they didn't use anything but the trailers, so there should be a disassembled trailer in the back of the truck (with wheels and tow bar removed). Dragon gives you parts for that.
There should also be the channels there that let you roll the trailer on and off the truck. Many gun trucks also left the tailgate open and put a kind of steel box there to increase the cargo bed space but still keep things (mainly ammo cans) from falling out.
Leave off the sight - they didn't use it in Vietnam as they were shooting people, not aircraft and as the classic once said "...you just don't lead them that much..." plus the sight breaks too easily. A real expert would tell you the small gas engine in the back and the batteries are WWII style, wrong for Vietnam, too. You can use the AFV Club's flash suppressors. Cap'n Mac - the "Tombstone" ammo boxes are wrong for Vietnam, too - they either used standard M2 feed trays with standard 100 round ammo boxes, or they cut down the trays and welded something there - I've seen cut down jerry cans welded there to hold more ammo, 40mm belted grenade boxes were very popular, too. This was throughly discussed in my build thread here:
http://cs.finescale.com/fsm/modeling_subjects/f/3/t/120370.aspx
And of course post some photos of your gun trucks - I have to build it some day, too!
Good luck with your build and have a nice day
Paweł