While not an "expert" -- I have set up a few Volvo rears -- both with new parts and in the case of my '69 144S I used a later used '73 rear w/4.3 gears--new bearings & seals--and a saved limited slip from a junked 265.
What you propose is not entirely silly. The differences in gear depth and lash is mostly determined by the differences in the housings---the machined parts should be close to if not perfectly identical. You'll need a housing spreader (try a Jeep guy). Use the shims from your rear on the used gearset and you should be pretty close. Take one set of bearings that you will not be using and grind out the center a bit so they'll slip on and off the shafts easily---that way if you have redo the shim configuration you will not be wearing out the shafts (the bearings are pressed into place--tight). Read a chapter on rebuilding rears with illustrations for interpreting wear patterns with the blue dye. Be patient. -- Dave