cover them in lots of penetrating fluid and leave it to soak (the longer the better). then try loosening the nut a little, then tightening a little, loosening, tightening etc. they should start to turn.
if there is a bit of the snapped studs remaining you might be able to get them out by welding a nut on and turning it out. if the stud is fully snapped off then an easy out might work but be careful not to snap the easy out. if those two methods dont work you'll proably have to take the head off and get them drilled out
