I don't like your chances of finding a Autocad or inventor file for a HQ-WB 1 tonner considering the HQ's where designed 40 plus years ago when all the drawing was done on the drawing board. I spent the last 4 years working for a truck body builder and the best I've been able to get are dxf files in a 2D format. Holden may have published a body builders guide back in the day so you could search for that. Are you trying to build a tray or are you looking at chassis suspension mods?
I've found it to be good practice to check any drawings I did of truck bodies against a cab chassis before the work was started as you never get 100% of the detail on the DXF's or PDF's that a manufacture provides.