First, drain down the coolant so there's none in the head. Use an oxy-acetylene torch with a welding tip adjusted for neutral or slightly oxidizing flame. Heat the center of the stud red hot, and keep the flame on it for another 15-30 seconds to allow the heat to conduct all the way through the stud. Now let it cool for an hour until cold. Don't quench it! The stud should now be softened and drillable. Get a piece of 5/8" flat steel stock and drill a 5/16" hole 1/4" through, and 1/8" diameter the rest of the way through. This will be your drilling guide. You'll have to either clamp it to the head with some large C-clamps, or else fabricate it in an "L" shape so that the pocket is centered over the stud and the long arm of the "L" fits between the rows of studs, By fitting between the rows, you can use the studs or bolts to clamp the drilling guide in place. Now use a 1/8" *left-hand* drill bit to drill through the center of the stud. Remember to reverse the drill! You'll have to stop and remove the guide to clear the chips several times. Use a drop of cutting oil on the drill bit. Left-hand bits are available from industrial suppliers (www.mscdirect.com etc.). If you get lucky, the stud will begin to unscrew as you drill. If it does, stop and remove the drilling guide. Once you drill through the stud, reheat it with the torch until its red hot, then let it cool until cold. Redrill the guide 1/4" diameter. Repeat the procedure using a 1/4" *left-hand* drill bit. There's a very good chance that the stud will unscrew while drilling. If it doesn't, you should at this point be able to peel the male thread loose and remove it using needlenose pliers.
Have patience! Heating the stud causes it to expand. But the expanding metal is constrained by the surrounding cool casting. This forces the stud to distort and expand lengthwise. After it cools and shrinks back to its original density, the diameter will be smaller and the metal will have lost its hardness. Heating it after there's a hole in the center will produced a more pronounced diameter shrinkage. Do not attempt to drill or do anything to the stud while it is still warm; until it cools completely it will have a tighter fit while hot than in its original cool state! Quenching the hot metal will re-harden it; slow cooling is required.
Using the drilling guide ensures that the female threads won't be damaged by the drill wandering off. Note that when drilling relatively hard metal (steel) embedded in relatively soft metal(cast iron), if the drill encounters soft metal, it will wander completely into the soft metal. You won't be able to control the drill bit direction by hand.
If you booger it, you'll need to remove the head and drill out the remaining stud on a milling machine, using a center-cutting end mill. The hole will have to be drilled and tapped to accept a Heli-Coil, or if its in a water passage, a plug will have to be screwed in and secured with red Loctite, and a new hole tapped in the plug. Its much easier to have patience while you judiciously use a torch and drill.
I don't recommend the use of an EZ-Out on a 5/16" stud because as the tool bites into the stud, the stud will expand and hold tighter.
As an alternative, and provided that the stud protrudes at least slightly above the gasket surface, you can place a 5/16" not over the stud and use a stick or MIG welder to fill the nut and weld it to the stud. Again, let it slow cool completely before attempting to turn the nut. If it won't initially turn, try applying torque while hitting the top of the nut with a BFH. Repeat using progressively more torque and hammer force. You can hook the box end of one wrench into the open end of the other to gain leverage. This will break loose all but the worst corroded threads.
Ken
