I wanted to do this to my 1995 945 so I went to a junk yard to see how sun roofs were put together. I suggest you do the same. There appears to be a major difference in the roof structure. in a non-sunroof car, there is a stiffing beam that extends across the roof of the car at or near the ceiling light. The beam did not exist in a sunroof cars I looked at. Also, you will need to cut out a section of the roof of the donor car to get the sunroof mounting bracket. you would then have to have this welded into your car (and have a new paint job). It appeared to be a major undertaking (and probably cost a min of 500.00). Did not appear to be worth it at this time.

Working with a sunroof is always a PITA, so keep that in mind. Also, you're going to need the headliner with the sliding sunshade, so figure you'll likely have to remove the windshield or rear window during the process and transfer that large, fragile item. But the actual glass/metal panel swap should be relatively simple.

I'd like to do this to my wife's '93 940, but the headache/benefit analysis does not yetmake sense...

I would compare the two cars that you have now to one another. The dimensions (LxW) are going to be the same. Open the roofs and look closely at how the roof panel is attached. Are the points the same? Are the tracks the same? Does the 95 have a sliding cover built into the headliner?

I think a carefuul comparsion will tell you all you need to know.