I pulled my factory single in single out on my 98 for the gmmg cat back. It sounds great with just my stock manifolds and cats but I am going lt and a catted y. Also if you look into gmmg they are not in business anymore the supplier of the exhaust though still is and they are called madhammer exhaust. Great guys and the system just slipped in.

 

I thought the same when my friend told me his '98 had a Moser 12 bolt. I figured someone swapped it out, which is what he thought too until he called SLP and they verified that's what his originally came with. Should have been just easier for GM to stick 12 bolts under the cars to begin with.