Sometimes you can get a pair of vice grips on the stub and back the bolt out. With the bolt being broken off there is no tension so unless the threads are corroded or galled should not be in there as tight as if the bolt were intact and tightened.
Another option if you have to get it out is to weld a nut onto the stub protruding out from the block. Generally a tack is all it takes, if they are corroded I will give em a little extra as the heat transfers through the bolt and helps with the process. This would likely require removal of the radiator and shroud depending on the amount of room you have.
Another option if you have to get it out is to weld a nut onto the stub protruding out from the block. Generally a tack is all it takes, if they are corroded I will give em a little extra as the heat transfers through the bolt and helps with the process. This would likely require removal of the radiator and shroud depending on the amount of room you have.