In concept, the HK Monaro was sketched in 1964 by Peter Nankervis including the full-width tail lights. An image of this is on the cover of the 1968 Holden Annual Report.
Of course, no single designer was responsible for the cars of that era. The HK was designed by a team while Joe Schemansky was GM-H Director of Design. Joe's first project at Holden was the HR Holden when he was asked to quickly revise the HD. Interestingly, Schemansky came from GM in the US where he designed the 1959 Pontiac; check that out in Google. Totally radical!
Early clay models were labelled Torana and it wasn't until late 1967 that the Monaro name was chosen after a region in New South Wales, Australia and was developed to help Holden win the Australian Touring Car Championship and specifically the famous race at Bathurst.
You could order the Monaro with a six cylinder motor but a 5 litre (307 cubic inch) Chevrolet V8 was available and the top of the range GTS model could be ordered with the mighty 327 cubic inch small-block Chevrolet engine.