The two most likely methods would be. One, if any of the bolt is above the surface (enough to see. but maybe not enough to grasp with pliers) file a slot for a screw driver. Second method, punch center, drill a hole into the bolt 1/8-3/16 or so being carefull to go up through the center (you won´t have much margin for error with the 3/16 drill, but the 1/8 snaps off really easy). A sharp drill and moderate (light pressure) will work better than pushing hard. Stop often and check to see if the drill is going fairly straight. Use an easy out, to remove the bolt. These bolts are fairly soft and relatively easy to drill. They rarley sieze.
Tightening, should be done with a 3/8 inch ratchet, holding the head not the handle. There is a torque spec for these screws. Overtightening, will often crush the gasket and cause leaks. Cleaning the threads with brake cleaner and using a light thread lock compound (loc tite), will keep them from backing out.
If the bolts have ever been overtightened, the holes on the pan may be slightly cone shaped, this will also cause the gasket to tear. Lightly hammering the cone (from back to front) will flatten things out some. Doesn´t have to be exact, just close.
Just for the heck of it, try taping with a small hammer next to the bolt (not to hard don´t ding the sealing surface), you might get lucky and the vibration will allow the bolt to back out some.
