to answer your question, the main difference is the heads, cams & intake. 99 is when they started with PI - 'power improved' heads. that is the main difference but there are other subtle differences, but that's the big one that makes the big difference
The main difference is horsepower (225 vs 260) due to the heads/intake manifold/cams. As someone said, the 99+ GTs were power improved (PI). You can slap PI components onto a 96-98 GT as many have done.
The main difference is horsepower (225 vs 260) due to the heads/intake manifold/cams. As someone said, the 99+ GTs were power improved (PI). You can slap PI components onto a 96-98 GT as many have done.
as this post said you can install the PI 99-04 heads,cams and intake onto a 96-98 motor. the compression is higher when you do the headswap on a 96-98 so you will make a little more power than a 99+ car. if you want too know more., do a search on PI headswaps it's a very straight forward swap
BTW if you are going to use a aftermarket head on a headswaped 96-98 i highly recomend checking PTV clearance ( piston to valve) have the new cams degreeed and check the PTV yourself ( or your builder) do not go by what anybody says will work.