What is the difference between gen 2 and gen 3 heads if any. I'm doing an engine swap in my 97 GT, I'm putting an 04 3.8 in and going turbo but I was going to save the head work until I do the turbo mod. I want to get it up and running NA first. Will the 97 heads work on the 04 or do I need to do the work on the 04 heads?