I have the Dominator EFI system. My Dominator ECU is located where the Ford EEC ECU once was (so the HP ECU will definitely fit), and the main harness fits very well. I don't like the ECU located under the front passenger seat (554-157/554-156) because of the main harness lump under the carpet. The first link below is how I installed it:
https://forums.holley.com/showthread...464#post100464 (Related Ford 5.0L Fox Mustang - Holley HP/Dominator EFI Installation Post)
https://forums.holley.com/showthread...845#post251845 (Park/Neutral Input − '87-'93 Ford Fox Mustangs, Automatic Transmissions)
https://www.holley.com/blog/post/usi...otor_behavior/ (Terminator X EFI Software, Better Control Of Ford IAC Motor Behavior)