easiest thing i found to do was DONT mess with the easy out because you may make the situation worse than before (cause you cant drill out an easy out, just push it further back into the head)
MY suggestion is to upgrade the two outside bolts to 10mmx1.25 studs (because replacing the 8mm with another isnt really fixing the problem is it
drill out the existing bolt thats still in there (making sure you go through the center, use a punch if you have to) then drill the hole out to the right size for the 10mm, rethread the hole with a 10mmx1.25 tap, clean it out, your done
thing is find a good 10mmx1.25 bolt/stud or none of that matters, now you just have a big hole with nothing to put in it
and make sure the 10mm bolt fits in the hole of the exhaust mannifold or header your putting on
putting sealant might last you 4-5 days before it gets pushed/burned out, plus you wont feel good about that kind of repair
im 17 (with no patience) did this by myself, i dont have much automotive knowledge or even a 1 car garage
cool thing was i did buy a lot of new tools, new header, and came out better $ wise than if i had went to a nissan dealership (or even the local scetchy garage) to get them to fix it