Thanks for posting those, Steve. The Monaro is interesting. The roofline and side windows reminds me a little bit of one of the '69 Camaro styling proposals that GM did before they settled on the final design, but it does have a little touch of Pontiac GTO-ness to it as well.
Thats an interesting image Jon, and yes, the family resemblance can clearly be seen. Holden was/is the Australian branch of General-Motors, and as such, it was common to have several American GM designers based in Australia at any one time, and likewise Europe, which is one of the reasons there are clear design links between GM cars the world over. Same was true of Ford.
In the case of the HQ Monaro, the design team was led by two American designers, Joe Schemansky and John Schinella. Furthermore, GM in the US would also send young designers abroad to gain experience.
The biggest challenge designers had in creating Australian coupes, was that, being a small domestic market, the same platform and the majority of the sheetmetal had to be shared with the equivalent four door sedan. So the HQ Monaro is a 2-door coupe version of the HQ 4-door sedan. Everything from the firewall forward, and the bottom tip of the rear windscreen back, are exactly the same. Only the centre sections are different.