Here's what I just installed into my 1998 GT.
NGK Iridium IX - Part No. TR55IX-7164
Note these are threaded all the way.
They work great. My gas mileage increased about 1-2 mpg.
Didn't really feel a power increase but it does seem to rev a little better after 4500rpm.
I got mine at -
http://www.clubplug.net
Ask for Frank (1-866-772-7584)
Good luck.
Ryan