YouTube my man...
View: https://www.youtube.com/watch?v=HnT1MtWOpHM
View: https://www.youtube.com/watch?v=7K_aZQfUHQY
Lots more on there. Search "1987 1988 1989 1990 1991 1992 1993 mustang remove cylinder heads" or "1987 1988 1989 1990 1991 1992 1993 mustang remove intakes"
Without knowing what intakes and what cylinder heads you are going to install its about impossible to tell you what the bottleneck will be. You really need to know what cam is in there as the cam is the brains of the motor and you can really donk the motor up with the wrong intakes and heads. You also need to match the mass air meter, throttle body, injectors and fuel pump to the equation as well. Will also need a minimum of a 2-1/2" full exhaust and at a minimum shorty headers.
If you just want to make a little more power then toss some GT40 or better heads on it with an Explorer upper and lower, 65mm throttle body, cold air kit, and swap out the stock fuel pump for a Walbro 190 lph unit and you will have some fun.