IPD sells overload springs which have a higher spring rate (about 20% stiffer) and will raise the back of the car a little. The same product may be available at local parts chains at a much better price. Another option is Volvo part number 552136-4, which is for rear rally springs that are 15% stiffer and will raise the rear by 2". I don't know how accurate those specs are, but it's a Volvo R-Sport part that was advertised in the 80's. I don't even know if those are available from the dealer any more.
Rear trailing arm bushings that are old and worn can cause the rear of the car to sag as much as an inch. Does your car clunk when going in reverse or forward from a stop? If you've got over 150K on the rear bushings, they're probably shot. Mine were in bad shape at 135K, and driving on rough roads a lot will shorten the lifespan a little. New bushings will help the ride quality immensely if they're worn out (MUCH smoother & quieter ride).
The front of the car can be raised a little with springs from a 200 series V6 or Diesel. Those engines were heavier, so the cars were equipped with stiffer springs. The springs won't be as compressed under a 4cyl engine, so the front end will sit a little higher (0.5-1.0").
On the subject of driving on dirt roads, do you have a splash pan? You can buy the plastic ones on ebay for about $30-35 shipped. IPD sells a 1/8" aluminum one for $165 if you need a little more protection from rocks or other larger obstacles. They are held on with 6mm bolts, although the IPD catalog says 10mm, but that's the size of wrench that fits on them. 6mmx20mm with a large washer to spread the load will work fine.
For new or used parts sources, see my links page below.