If you really want it, it can be done. My friend just retired his 88 244 that had this done 5-6 years ago. The shop put both cars side by side and just cut and spliced the roofs together. Totally factory looking job. Only thing to keep in mind is that you probably want the seats out to avoid welding burns. There will be some filler needed and of course the roof at least needs to be repainted. It's totally possible- but only you can decide if it's worth it to you. You might consider finding a much later donor car to get a newer mechanism, but as you note, you've got the complete car on hand. Any roof from a 240 up to 93 would work, BTW. You are correct about the scale of the job- essentially almost the whole roof has to come off. Fortunately in the back corner VOlvo has a built in seam to hide part of the repair (under the little V trim) if you cut that much out.
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Rob Bareiss, New London CT ::: 86 244DL, 87 244DL, 88 744GLE, 91 244: 808K total