HK Monaro was always a banjo when the PTC of 307 and auto was optioned, unless you also optioned a 3.36 rear axle then you got a Salisbury. HK GTS was the same early on, it wasn't until August 1968 that 3.36 became standard in GTS with 307 auto, prior to that the rear axle would be either 2.78 or 3.08 nodular LSD banjo. Once 3.36 Salisbury became standard in GTS with 307 auto you could still option 2.78 banjo if you wanted but not 3.08 banjo, in that case you got a Salisbury.

Note that the speedo cable has no "V8 position". The position you are talking about in HK is for V8 Powerglide, M21 or M22 4spd. It is also a remote possibility the car was a standard GTS but optioned with a Saginaw 4spd. This would have the extra speedo cable hole regardless of the engine. A 6 cylinder with Saginaw 4spd in HK is very rare though outside of ute and van (where it was the standard 4spd). So my guess is you have a V8 auto HK Monaro of some sort. If you email me the ID details via this site (email not PM) I'll be able to help you further.

Does it have 5 or 6 inch rims

Look for extra spare wheel brackets on driver side rail in the boot. If it is a July 1968 car most likely had 5” rims.

^^Same as mine too.