Found this while searching ebay. Looks like they have the mounting bracket for the 99s too!
http://cgi.ebay.com/ebaymotors/FACTO...QQcmdZViewItem
These are not mine

 

Wont work on a 99. They dont have the mounting bosses on the valve covers.

 

Oh well, still not a bad price for just the covers. Don't these usually sell for around $100

 

i think around 130 new. you could make them fit on a 99 if you wanted to.i put a new engine cover on my 00 which came with a cover that has 2 mounting bolts instead of one. i bolted it in the front hole and it fit fine just a little loose. if you put those side pieces on there then it should be pretty sturdy


edit thought you were talking about the center cover
i guess if you wanted to you could change the covers

 

I paid about $85 for all three pieces and the clip thingy from GM parts direct.