Maybe you could use the complete 240 rear axle assembly all the way out to the brakes.

This would entail a lot of fabricating and welding of mounts for the suspension, driveshaft adapting, brake plumbing and other details. At the end, you'd have rear discs, the later bolt pattern, stronger axles and a choice of higher(lower numerical)final drive sets.

I'm curious about the aftermarket wheels you mention. Are they the old Unilug style? If so, be sure you can find appropriate inserts to use.I'm doubtful that inserts with Volvo's later bolt pattern will be easy to find.

What about just living with two different bolt patterns? Not optimum, I'll grant you, but much easier. For the spare, you could keep a cheap wheel adaptor with your spare tire to use on the noncompliant end of the car.

If this were my choice, I'd want the 122s 5x4.5" bolt pattern all around, as opposed to the later pattern, simply because of the huge selection of stock and custom wheels available, in all diameters/styles/offsets.