He's going for wider, just not the monster meats that you have to run to fill out the guards. It'll be a regular street car, so he doesn't want to run 10 inches on the rear. But there's a VR diff (with discs and LSD) in the spares pile, which gives him 2 inches of extra width.
Running a VR diff brought up the question of "can you run VR on the front as well?" I know the K-frame fits, but not what happens when you use the tca's, struts etc. Didn't know if it completely destroyed any hope of getting decent camber, caster or toe for instance. Because if we can use VR struts, that goes a long way to sorting out a decent set of brakes as well. Anyone got any experience with it before we start ordering mail-order parts?
"Going for wider, but not going for wider". Why no new wheels? Would be the absolute easiest way to help achieve whatever he's going for and wouldn't have to run stupid sized wheels. Even an 8" or 9" wheel with the right offset (choosing your offset is really useful for this) shouldn't foul at all. Really useful site -
www.willtheyfit.com
I don't think you're going to get the look you're after without planning your wheel offset to the mm and cutting into the guards. You WILL need new wheels; you can play around with control arms and diffs all you want, but they're only a fixed amount wider and you can't change those dimensions like you can with offset or wheel width, there is just no way around it. See this thread for a couple of VS examples, even just look at the front wheels (some of them have shortened diffs for deep dish, so ignore that) -
https://forums.justcommodores.com.au/threads/vs-sedan-and-ute-modern-fitment-thread.273508/
Gotta remember that many of the flared early Commodores were largely race cars with lots of dollars and heaps of body work. You can put flares on but you still have the original body work to foul on, it's the same thing that stops people running a setup like this without extremely stiff suspension or just running the car sky high so it looks like it's on stilts. It's the reason they sell wide body kits that pretty much require you cutting and re-welding the rear inner arches and all quarter panels. IMHO you'd be better off rolling the inner lip of guards and not bothering with flares, otherwise your car will essentially look like this -
https://forums.justcommodores.com.au/threads/new-project-flares.249979/
The VR diff is more like 1.5" wider instead of 2". Running a VR strut will not change anything in relation to running different brakes, it's the knuckle that counts. You can run the VR/VS control arms, but you will also need to change the knuckle to a VR/VS knuckle (which is better anyway because it uses a clevis) and then figure out if you can use the VB-VP strut top on the VR/VS strut so that it will bolt into the strut tower. Unsure how this will affect your geometry and turning circle. Other people have used an S13 coilover. A VT/VX control arm can also be run (which is like 20mm wider or something) if you use the VR/VS knuckles. Thing to remember here is that it will increase track yes, but it will increase a decent amount of camber and the top of the wheel will not come out as far as the bottom because it's a McPherson strut and the top of the strut is fixed with regular shock absorbers. Doubt the factory camber adjustment would completely cover a track increase that big.
To really be honest I think you should buy wheel spacers to mock up and test drive whatever setup you're going for since your friend only seems to be focused on track increase. If you are absolutely dead set on not buying new wheels doing this will pretty quickly show you the limitations you will be facing especially for body work and whatever limited size factory part you want to swap in, particularly depending on how aggressive you want your fitment to be and what I assume will be largely stock suspension (King Springs and stiff shocks are no better, in fact they'd probably be worse).