Mach 1 Prochamber Question

Hello! I come venturing from the 94/95 area of the thread and I'm looking to buy my brother a mid-pipe and MIL elmininators for his birthday. He likes the sound of a prochamber and the SLP-1 catback, but I'm not entirely sure which part number I need for his car on the MAC front.

I checked out their website and I see two options:

PC9900 which is 99-04 GT/Cobra 2.5" Pro Chamber

or the

PCM9924 which is 99-04 2V/4V Modular Pro Chamber with Off Road/Cat legs, to stock.

The second is nearly 600 bucks and the first is half of that. He is keeping his headers stock so, what do I need guys?

Thanks!

EDIT: He doesn't want cats, if that wasn't caught on to by the mention of MIL-elims.

Thank you, and it seems you are right. I e-mailed them as well because I was curious and it is indeed the less expensive one.