I doubt this would have been the case. At this time, Australia had been involved with body-building for a very long time, and as previously mentioned, because of the local tariffs, locally produced bodies were in favor. Australia had a very developed body stamping/building operation going on.
What hasn't been mentioned so far is that a lot of the 1930s-50s American-based Ford/GM/Chrysler utes that were built in Australia were based on the tourer/convertible X-frame chassis, purely for the extra strength. For example, in Australia, the late 50's ute chassis (Mainline), used a Sunliner X-Frame chassis. I would think that if the OP looked at the chassis on his Chevrolet ute in question, it would be one of the reinforced X-frame chassis.
The tourer/convertible chassis were used, as they provided extra strength, whereas the sedans/coupes contained their extra strength in the roof structure.