OK. I thought as much that it would come down to assembly.
I worked at GMH Fisherman's Bend back in 79-82 during their end of year shut downs. There I saw how the panels went from drawing to clay moulds and then to panels. Of course each plant would have different methods of assembly due to different equipment, training etc.

My understanding was all 'master' body panels were done at FBend during this time because the other plants did not have foundry lines. I'll have a chat to my uncle whose memory is fading now, but he worked at Holden in Fishermans Bend during the FB-HG years.