I used factory studs. I had to shorten one on the pass side to make my Kooks fit. Studs are superior and easier to use. Buy new m8-1.25 metal locknuts though! Pulling all those studs and trying to start bolts without damaging any threads was too much of a risk for me!
Not sure what difference it makes, but the OEM studs are fairly long and may need to be cut down for clearance on the lower bolts, depending on how the tubing is run on the headers. Bear in mind, you need enough room to start the nut as well. I used stainless steel header bolts on mine.
I used a Stage 8 header fastener kit on my 2008 when the FRPP shorty's were installed, that was 4-5 years ago.........maybe even longer........no issues.
Stage 8 on mine. These mod motors don't rattle like the old push rod 5.0s! Back then you had to constantly tighten up the header bolts! My kooks have been on for over a year and not even a hint of exhaust leak! The old studs came right out for me and the angle at which the new bolts go in I never felt concerns of cross threading.