looks like deanfj55 doesnt login to mud much any more - pm me if you want to know anything about how we did this conversion.. Basically of all the accredited engineers in our area (about 15 or so) - only 2 were interested in certifying a dual cab conversion on an FJ55 - the rest said it was too old or just not interested
Anyway - our engineer wanted 75x75 RHS as an interior hoop in the back - with chassis bump stops underneath.
We extended the 75x75 RHS theme by making an exterior hoop and then the outer part of the tray also 75x75.
We used 3mm checkerplate steel for the tray floor and had just enough to make the sides out of a 7ft x 4ft sheet.
I recall that we looked at normal landcruiser / patrol etc trays that we could shorten - but they were all too wide for the piggy.
Originally we wanted the tray to align with the wheel arches (where we cut them) - but we found we needed to raise the tray by 25mm to get fuel into the tank. We really need to raise it a bit more because its not easy to get a full tank of fuel in there - particularly with a high flow diesel bowser.