spray some good penetrating oil on the threads and let it soak for a good while, then you can run a die (thread chaser) down the stud, or run a nut and keep backing it off to get the crap out from between the threads.
Soak her again if she's not cooperating. Lastly buy some aftermarket headers so you can use replaceable bolts instead!!!!
Unless you want to auger those studs out of there!
1997 Sport - Work in progress:>
MP BB 304 Teksid 4v- w IC'd / Ported M112
Eaton pulley puller (VMP or BF preferred)