It is in no way an "add-on" job to your existing harnesses or instruments. Putting an Street Glide/Electra Glide fairing on a Road King will require ditching the Road King main harness and going to the Glide's main harness instead. That means all the instruments are in the dash. So you'll lose the Road King speedometer and will need a dash from Street Glide or Electra Glide. A few different styles to choose from there.
By the time you buy a main harness, dash, instruments, fairing, radio, speakers mounting hardware, headlight, windshield etc etc etc you're looking at a very expensive conversion. Changing dashes on the tank might mean a new seat too; depending on the dash you choose. It's not that this isn't a do-able task. It is. But to me it wouldn't be worthwhile.
By the time all the parts were bought, fairing painted to match the rest of the Road King and spent a lot of time making the conversion it's very likely you'd feel money and time ahead to have just sold the Road King and bought a Street Glide or Electra Glide.