One can run a fairly thin slider style long-stud spacer arrangement. Material choice for the spacer, stud and lug starts to be pretty important. Are those longer studs going to stretch more and cause more unpredictable clamping force to keep the nut still and wheel on? Are those spacers going to have enough material to sustain repeated abuse on a lever-arm extension of the hub? Is the spindle going to be strong enough to cope with spacing the wheels out far enough and the moment arm and additional weight inertia on that moment arm (often minor) and we can go on and on....
Using a good quality stud and lug with a good quality spacer, I've seen wheel centric and hub centric spacers as thin as 8ishmm to fit a volvo hub correctly. Whether or not they are as strong as OEM, I do not know...some further analysis would be needed. One could go thinner if one made a spacer for a wheel with a larger center bore than the bore on an OE volvo wheel or OE volvo hub.
I'd just re-machine the wheels instead of use any spacers at all in some cases....many times sufficient material exists to safely face some off the wheel mounting and re-work the center bore. If the center bore is too large on the wheel, sometimes pressing a machined centering ring (usually with interference fit with the wheel...wheel in oven and ring in freezer type deal) into the larger bore to fit the desired hub is a safe and logical option.
There are lots of variables, but also lots of safe solutions. There are also a lot more not so safe solutions. Consider paying for a well recommended machinist or engineer or both....they are *just* wheels that *just* control your brakes and steering actions bwahahaha This is by no means an impossible task if the goal is to push the wheel out 2-4mm...making spacers, modifying hubs, making hubs, making wheels, modifying wheels are all commonly used possibilities (one can always do other things to the car or suspension of course)...all safely, cost effectively, (backwards) compatible with the existing car/easily reversible, easily serviceable and/or factory serviceable and light weight...