With the VB/C/H/K/L Commodore your biggest problem is there’s no venting between the boot & the cabin worth mentioning, which means any boot install you do wont get much bass into the cabin where you want it. All it will do is make the boot lid/number plate/rear light/fuel tank rattle and sound like shit.
First off, do you have two rear speakers OEM fitted ? They were optional, so you might not have them.
If you do, I’d suggest mounting your 8″s in those holes on both sides, and rebuild the parcel shelf from 12-16mm thick MDF. Dont fit rear full range speakers.. After all, you dont sit in the back seat and drive 😉
Alternatively, you could get a pair of 6″x9″s or Kenwood 7″x10″, and mount them into the OEM holes, the motors will fit thru if you rebuild the parcel shelf from 12-16mm thick MDF. You can also leave the 8″s out, as 6×9/7×10 generate the same bass frequencies as an 8″ or 10″ woofer would.
Your final option is to cut two 8″ holes in the metal under the parcel shelf and mount the woofers there, then rebuild the parcel shelf from 12-16mm MDF (notice a trend here ?). But be warned, its bloody fiddly & difficult to cut those holes due to the cramped space you work in. Also, that area is a structural member of the car, so cutting it decreases body strength. Hence it is technically illegal and you can be defected for it, or rejected for rego.. so if you do it, its at your own risk. I cut a 10″ hole in my VC and never had troubles, but that doesnt mean you wont.
So those are your sub options. Because the subs you have are free air, you dont mount them into an enclosure. Just rebuild the rear shelf from MDF and bolt them to it, the boot area will act as the enclosure for them. Same if you use rear full range or 6×9’s, bolt them to the MDF board. This gives the speaker a firm baffle which will improve their efficiency & clarity (less wasted energy in rattles).
For your front speakers, VH had a single indash speaker IIRC, same as VC. Forget about that and put them in the doors. Make up a new door card from 6-9mm MDF & trim it. If you’re careful, you can remove the OEM trims and attach them to the new card. Alternatively, make an MDF baffle to attach to the door car. Make the baffle around 3-4″ bigger diameter than the speaker.. so use a 9-10″ baffle ring for a 6″ speaker. For tweeters you dont need such a strong mount, the masonite door car will be fine. Mount the baffle ring behind the door card (a dab of liquid nails or even BluTac will be fine) and screw the speaker thru the door car into the baffle.
There’s metal frame parts behind the map pocket area which might require cutting or bending. Again, this is an At Your Own Risk thing. Mind you, I used to work in a Holden dealership where we’d cut the frames and mount speakers.
Finally, replace all the noise surpressors (there’s one mounted on the alternator, one on the ignition coil) in the engine bay. They’d be over 20 years old by now and completely stuffed.