After market headers flow better than stock and require changes to the pcm tuning to get peak performance out of the car.
Since you don't mention what year your car is, and the year makes a big difference in tuning software available, I'll do a run down. Of course you can always have a tuner do the work for you, but I'm assuming you wish to go the do it yourself route.
If you own a 94 or 95 obd1 car, You can get a cable that hooks your car's dlc to usb of laptop, tunercat obd tuner, and DataMaster to log the sensor data, for $280 or less.
If you have a 96 or 97 obd2 car the tuning software gets much more expensive. Just for jet dynamic spectrum tuner, be prepared to pay about $500, other obd2 tuners run that much and more. You will still need the cable to hook dlc to usb of laptop. That will be $60-90 depending on where you get it. And you will need a obd2 scan and data log program similar to DataMaster, which I have no recommendation for.
For a 93 obd1 car, which has a removable chip, rather than a reflashable one like 94-97, you will get the same programs that 94 & 95 used, plus the addition of a chip burner, its program and chips to burn.
For sending your pcm, or ecm on 93, out to a tuner, members here use www.lt1pcmtuning.com www.pcmforless.com
And if you go the tune it yourself route, you must obtain the knowledge of what pcm settings to change, and how much to change them. I suggest reading as much on the subject as you can. The lt1 tuning site has some good info, as well as, other places on the net.