you're looking for gt40 heads, and I don't know about the 99's, but i know that most of the 5.0's are on the explorer's. If it's a 5.0, you're probably fine. Search for the install, and you'll probably find plenty of info on it.
The 5.0L Explorer has GT-40P heads. They are not the same as GT-40 heads found on Cobra's. They are better then stock E7 heads found on GT's. The P heads require a special header. MAC makes P Headers. You can do a search. Many people on here are using P heads with great success.
i dont remember the cutoff year but it MAY be 99.....As long as it has a 5.0 your good. Any explorer made after April in 97' has P heads...which IS what you want. Also dont listen to internet myth about p heads needing special headers.....I have them....you don't.
Also, they provide a HEALTHY increase of power and when combined with a good intake and cam, your greatest wishes will be realized.