A good 87 - 93 'stang will pull about 205 to 210 RWHP with only headers and exhaust mods.
I'd recommend going with suspension mods before you start adding more power to the car. I have a 1989 hatchback, and it was running 13.5's with the stock engine. At that time it had a T-5, steeda upper and lower control arms, tokico 5 way adjustable drag struts and shocks, sub frame connectors, and a detroit locker with 3.73's, MAC equal length shorties, and a 2.5" catback system along with an off road 'h' pipe.
Once you get the suspension set up so it can handle the power without jumping around or flexing you can start in on the engine. The HO 5.0's are about the easiest engines out there to work on when it comes to adding additional horsepower. Not difficult at all to get at least 300 RWHP, N/A..