The book David refers to is Armor of the Vietnam War (1) Allied Forces
by Michael Green & Peter Sarson, and not Vietnam: Armor in Action
by Gordon Rottman & Donald Spaulding. The former has a few good photos of different ACAV interiors on page 42, taken through the open rear ramp, but the latter doesn't.
But really, what it comes down to is to remove the commander's seat and everything associated with it (the pillar and the "shelf" for him to stand on) and then pack lots of ammo boxes, a few water jerry cans, and assorted other boxes and stuff into the vehicle in a fairly neat way. A row of ammo boxes under each bench, and another row on top of them, plus a pile of ammo boxes against the engine firewall, puts you a long way to having correct interior stowage