Agreed with Doug Reid that the GDL-82 needs the AV-74 due to the 978 MHz frequency. But otherwise it’s a pretty straightforward install. (Mine is in an LSA, so no guidance on the PA-28.). As for the RG400 cable - there are folks on eBay (and Aircraft Spruce) that can make the cable to the length you desire. I did that to save myself time and make it an (almost) plug-and-play solution.