Have a 99 Gt with the auto trans mission , I can get a great deal on a built trans from a 03 Gt, was just wondering if they are interchangeable.. Thanks....
At the very least you can expect to have some speedometer calibration "issues". Expect that the number of teeth in the OSS reluctor wheel will be different. This can be corrected in the same way as a rear end gear change.
The 1999 will have a 4R70W where as the 2003-2004 will likely have a 4R75W. I honestly don't know if they are fully "plug and play" interchange. For a list of true "plug and play" transmissions try searching car-part.com.