Don't use high temp sealant unless you want your car to stink like hot rubber for an hour after you turn it off. All you need is a genuine subaru gasket on each side, they are like 13 bucks each, and have a metal seal and composite flange.
If you're gonna drop your headers to change gaskets anyway, clean the threads and make sure the studs are threaded up far in the block. Don't use any anti seize unless it's very high temp (1000C+) or it will stink and smoke.
Tighten the bolts to (35Nm? check your years service manual) then after a few drives tighten them again. Alternate between cylinder banks when tightening.
Good luck
