What trans and pcm are you using? 98 Pcm cannot work with a 03 trans and Vs versus. You will need a 2003 pcm. Also if it came from a lesabre Id think it has a 3.05 Gear ratio and not the 3.29 in your GT(not 100 percent sure)

If your still using the stock trans and stock PCM you need to swap out the maf sensor to your 98 if your using the 03's. They are different and the maf sensor will play havic on trans line pressure