If you have a wire welder, you can weld a flat washer on the top of the stud.On the block, use a washer with a smaller dia. hole the the stud, then weld a nut to the washer. This is going to give you something to get hold of to turn the stud out. Also by welding to the stud, you are putting some heat in the bolt and that will help break the rust free. As the nut is cooling off, gently tap on the top of the nut with a hammer, again this will aid in freeing the rust in the threads. I hope this helps.
This method has worked very well for me in the past.