I’m in the middle of using a 93 240 sedan donor car to rebuild a 79 245GL...you can see the thread here if you’re interested.
https://ozvolvo.org/discussion/9202/gregs-79-245gl-barn-find/p1
As mentioned above the upper plenum panel below the windscreen changed shape in 1986 and the bonnet changed shape then to match. If you’re doing a complete re-build with rust repairs and a re-spray on your 242GT and want to update to the 92 front end, the best option would be to unpick the 92 upper plenum panel and weld it into the GT...especially if your GT has rust around the bottom of the windscreen. That way you can have all the parts match up.
Swapping over the suspension, fuel system, wiring harness etc. is pretty straightforward. Most of the new parts interchange easily onto the older car, with only some minor holes to be drilled and/or repositioned. I assume you’re going to swap in the later engine, fuel injection system etc? Another good project - please start a build thread on it if you are so inclined so we can follow along!