fairly certain they have different PCD ( pitch circle diameter ) ( stud patten )
also couldnt be sure if the spring mounting width ( on the axle tube ) is the same ! just checked on my tonner and the outside to outside of the rear spring is 1230 mm
something in the back of my brain says HG pcd is 5 x 4 1/2 inch and HQ is 5 x 4 3/4
im sure someone will tell me if im wrong

You are correct, the stud pattern is different. Not sure about the spring mount. I have to measure the ones in my HG. Might be able to use them, maybe not. I would like to swap the brake drums over so I don’t have to buy new wheels.

HG is 5 x 4 1/4”.

The drums are basically the same between HK/HG and HQ/WB but as above the stud pattern is different and the centre hub is a different diameter. If you put HQ-WB backing plates on an LH-UC Torana the HK-HG drums go straight on if that makes sense, as the drum dimensions are the same except for stud pattern and centre bore (Torana and HK-HG are the same in these). However due to the HQ centre bore size and stud pattern the HG drums won’t fit the HQ diff.

If you get the mounts changed on the HQ diff it will go in the HG, just have to use the HG handbrake cables and a V8 tailshaft probably with an adapter uni with 1” cups on the tailshaft and 1.125” cups on the diff (as cab-chassis has a bigger uni). That or a HG GTS350 Orange tailshaft. What you will have then is HQ rear stud pattern so you need to run HQ pattern rear wheels with more backspace to suit the wider diff. HQ rotors and calipers will bolt straight to HG stubs too, so you then can run the same stud pattern front and rear.

Check with an expert on what rear cylinders to run though. Might need a HQ prop valve in the rear circuit. Dr Terry will know.

Here is a HQ-WB diff modded to suit a HK-HG already. Might even be worth buying as mounts are done and has a tailshaft with it to suit a Chevy engine with Saginaw/TH350/Powerglide.
https://www.gumtree.com.au/s-ad/croydon/engine-engine-parts-transmission/holden-hk-ht-hg-v8-diff-with-drive-shaft/1264750324?utm_campaign=socialbuttons&utm_content=ios_VIP&utm_medium=social&utm_source=com.apple.uikit.activity.copytopasteboard

No. HD-HR are the same as HK from memory, but HT and HG went softer in some models with a larger front spring bush. HG all have the larger front spring bush except manual GTS350 if I remember correctly.

HQ-WB are very different springs.

To be honest you are probably better off buying that one in link I put up. You are going to probably spend more than $1200 starting with a HQ tonner rear axle. You are up for:

Cost of rear axle. A 3.55 HQ cab-chassis axle assembly is worth at least $500 today.
Weld on HK-HG mounts or move the HQ mounts and buy U-bolts and HQ-WB lower shock mount plates.
Straighten housing which means disassembling it.
Welding on HG brake hose lower bracket.
Make up brake lines to suit.
Reco the HQ brakes and fit HG handbrake cables.
Get a tailshaft made, a $450-500 exercise.