Might be the spacer plate between the IAC and throttle body. I have a 1990 5.0 F150 and changed the IAC and had high Idle, it was 2000 - 1500 because the spacer plate was put on backwards, on the 5.0 the plate has holes on one edge, and the holes should face forward on mine, so that the side holes feed the large incoming port to the IAC and not the small out going IAC port on the TB. You might check yours or try it the other way, incase its flipped.