I just replaced our SL40 and Flightcom 403 intercom with a GTR 200B. The panel opening could stand a few passes with a Swiss file to make it just a smidge taller. I removed the old intercom and covered the holes with a stick-on CO indicator for now -- we'll do something a little less "ghetto" later. It is not a simple plug-and-play installation, nor do I think there's a wiring harness you can buy to do it, unless you want to pay someone $$$ to build you one. I made a spreadsheet detailing all of the wiring changes and worked through them with a handful of D-sub pins (almost all of which came with the GTR 200B) and a good crimper and stripper. Took me a couple of evenings to get it all done, and it was well worth the time and effort.
The new radio's design is 20 or more years newer than the SL40, and it shows. SO MANY more features, all of which are easier to use. Better audio, better intercom, better display, better everything. It's WELL worth what you'll pay for it, especially if you remove and sell your SL40.