Johnson has used a Tube Screamer distortion or Fuzz pedal periodically and interchangeably, thus it's the best way to replicate his sound in terms of distortion.
Couple that with your amp and here's how you can get the closest to his tone.
Tubescreamer or Fuzz Machine SettingsFor this example we'll use the "Fuzz Machine" from Garage Band which has all the settings we'll need to identify and utilize.
Eric's FXIbanez TS808 Reissue Vintage Tube Screamer
Electro-Harmonix Deluxe Memory Man Analogue Delay
Dunlop Fuzz Face