All the above suggestions assume the thread is in good shape and not galled or otherwise compromised. If the thread is seized, it's probably going to be necessary to drill the bolt out a little at a time after getting a center punch dimple in the exact middle of the bolt. I'd suggest renting a magnetic-base drill press that holds itself to the top of the block for that operation, since even a slight misalignment of the drill bit will result in a junk block. Drill the bolt out about 1/32" at a time until you get to the minor diameter of the thread, then use a tap to clear the remaining residue.
If the thread in the block is severely damaged, drill and tap the hole for a heli-coil while you're set up with the drill press. Good luck!
Jerry
