Hi, David and welcome to the world of puddle jumping!
To correctly determine your unit sign, you also need to find the divisional sign, usually located on the opposite front guard. Red and Light Blue is an odd combination for a WW2 vehicle sign, it could be RNZAF. Could the light blue be faded dark blue or green? Are the colours split horizontally, diagonally, or vertically? Any possibility of a picture of the tac sign or your truck?
C8AX's are more common in NZ than anywhere else in the world, I know of a couple of restored ones in the South Island and I'm sure that there is a few more in the North Island. For a good look at a rear deck, try contacting you local branch of the NZMVCC.
Where are you located?