Hi, I'm also working with a '99 GS.
I just removed my stock airbox and in looking at the design have decided to modify it so a 16" cone filter will fit. (Probably soon, and I plan on taking pictures) There is a 4" duct inside of the huge airbox that gets air from behind the headlight. Most of the box is an air cavity in which the pcm computer is located.
I didn't plan on extending a duct to the foglight area. But I do plan on gutting all the extra plastic walls. I plan on putting the computer in the same stock location (it fits well in the bottom of the airbox (I havent seen an older Regal or GTP airbox, but they must be radically different as my PCM is no where near restricting airflow) ).
My question to anyone that might know or have an opinion: Is it better to have a single inlet OR not only have that forward inlet but also a few blow holes on the side?
The whole point is to make the filter less restrictive but is it better to give it only fresh air or allow it to get air where it wants?