If a bolt shears, then a proper fix would be neccasary for a decent seal. Depending on which bolt shears and where, ie, in the head or the manifold to exhaust connection, access is very limited and head removal may be the only option. If you are going to go down this route, take the car off the road for a day or two before doing the job and completely soak the bolts and studs in penetrating oil. Keep applying the oil every few hours and let the oil do its job. BUT, there are no guarrantees.