What's the possibility you are trying to put together the wrong parts or were they already installed ?
Exactly what I suspected. Aftermarket parts not meshing with OE.I found out that AMS downpipe flange is larger than stock. That is why the OEM stud perch goes into the downpipe flange. The FactionFab studs have a larger perch. This is not an issue with stock downpipe flange. As stock rests on top. But will not work with AMS downpipes or some other aftermarket downpipes. Also verified this from AMS as well as FactionFab.