Which one stripped? The one that goes straight up and down or the one that goes in at an angle? A heli-coil is a theoretical option. There isn't a lot of torque on those bolts so it might work. The lower intake will need to be removed and I would not use the gaskets as they have been crushed by the over torqueing. The heli-coil will come as a kit (drill bit, tap, coils, and install tool) simply drill out the old threads, tap the hole, install the coil using thread locker, reinstall the manifold. The only thing that might be a problem is the coils have a little tang on the bottom used for installation. If the bolt goes further than the tang than you might be out of luck. Its already broke so whatever you try really can't put you in any worse of a bind, can it?