There is a how-to on rear shock replacement at VolvoSpeed that might help:

http://www.volvospeed.com/Mods/Bilstein_rinstall.htm

However, based upon your description, I believe that you are referring to the shock mount bolts, not the bolt on the shock piston itself. These go through the floor into a captured nut (really just a thicker piece of metal, maybe a quarter inch thick, welded to the body and threaded to accept the bolt). It sounds as though your original captured nuts are either gone, or the threads are stripped out. Regardless of which it is, I think your easiest course of action is to use a flat washer and a nut on the underside of the car to replace the damaged or missing captured nut.

It sounds like someone may have put a nut on the bolt underneath the captured nut at some point in the past. That would explain why you were unable to loosen the shock mount bolt. Regardless, a nut and washer on the underside should provide equivalent security to the original design, it will just require a second person under the car to hold the nut while the bolt for the shock mount is tightened.

Phantom