Those are not fun. They will rust/corrode where the bottom is in the water and act as a stop as they turn up. If you get them to move, keep soaking and turn them up and down trying to get a bit more of them each time. If they stop , that's when they break if you turn more. New ones should be just hand installed before you put the head on. I put permatex #2 in the threads of the hole and stud both. They like to leak antifreeze up the threads and out on top of the head. Tapping with a hammer on top of the stud can help to break them loose.
Do they need to come out?
You might check the block for flatness with a straight edge. If good, maybe leave them alone. You said you painted the block so I'm assuming you are not having the surface milled.
I would not tap the threads out with a tap unless you have to. It tends to seat the new stud deeper in the block and you only get half the nut threads engaged. Ask me how I know this. Maybe just chase the threads with a thread chaser.
Another tip is to use two nuts locked together and then a air wrench to turn on them. It puts a straight on torque on the stud like a T hand wrench does. Off to one side with a ratchet puts a side force on them which isn't helpful.
Fingers crossed clear around for you Tom!
