Doing a great job there, Wayne. One of the more unusual Australian vehicles and because of the construction, very, very few survive.
Your vehicle is, indeed, ARN 67654 and is a lone entry amongst other types. The engine number was listed as SR3836204, and curiously, the chassis was listed as 2842130071, whereas the plate clearly shows 2842100071 - which only goes to prove the Army clerks transcribing the information didn't always get it right.
It was taken on charge in Western Australia, and probably remained there for its entire military career, being declared for disposal at 5BOD on 3 December 1953.
The Formation sign of the flying black swan on a gold background was the post-war insignia of Western Command introduced in the late 1940s or early 1950s: the earliest date I have for its use is October 1952.
Western Command in the Second World War originally had no formation sign as such, but in 1944 the Magpie on a Boomerang sign was introduced. The 'W' sign you mention was part of the Unit sign, rather than the formation sign. Its earliest use was as the area designator for Lines of Communication units, where it appeared in the middle of the 2 inch wide white line across the bottom edge of the Unit sign.
Later the 'W' was used in place of the numerator for units with a territorial affiliation, eg W over 707 was the WA Echelon and Records Section. The territorial letter system was introduced in 1945. Units without a territorial affiliation or 'non-standard' units continued to use the numerator/denominator system of numbers.
You mention that the nose section is a replacement, and I'm curious as to how you came to that conclusion. The body and the nose section were most likely manufactured in different plants (nose at Woodville, SA, and body in the Special Body Plant at Fishermans Bend, Victoria), which would account for the differences in paint.
Again, nice truck, and being nicely restored: a real credit to you.
Mike