Curious about the internal structure of these. Did they utilize the conventional bridge-truss system that was commonplace in the 1950's and earlier for cab units, or were they early forerunners of cowl units where the locomotive frame was load bearing and the full width body just provided protection from the elements?
Good question; I don't know. None of the small number of pictures I have (in various sources) show diagonal truss members showing through the ventilation grills, which would be a good sign of "bridge-truss" construction, but they might not show.
An old (1970s) article on them in "Railroad Model Craftsman," written by Win Cuisinier (one of the best-informed writers on dieslelocomotive history!) just says that they closely resembled Alco FA/FB-2 in carbody design and equipment location, which isn't as explicit as we would want to give a definite answer to your question, but certainly doesn't suggest there was anything radically new about their carbody design, like a cowl arrangement.
GE's Australian licensee, Goninan, used a very similar carbody on a batch of locomotives (Class 43) for the NewSouth Wales Government Railway, in roughly the same period that NSWGR got conventional EMD (Class 42: six-axle F-7 variants) Alco (Class 44: "World" or Dl-500 type) cab units.(*)
Bottom line: I don't know for sure, but if I had to bet I'd go with truss design.
(*)There's a weight penalty with cowl and load-bearing frame design, which I doubt NSWGR would have been happy about, but this is also not conclusive, since they also got hood-type units.