as a former northeast boy, we were fed a special Gerber baby food that consisted of rust in a library paste binder. Awful stuff, but it taught us how to remove frozen bolts and studs.
There is no reason to ever break a stud or bolt if you are careful, take your time, and do a little at a time. Only work during your most patient time of the day, typically the AM.
I have a slightly modified method from one above. Yes, heat the nut or stud with an O/A torch until it is cherry red. Do not play around with propane or Mapp gas, that stuff is a waste and will give you a false sense of security and then you WILL break something. Get a friend over with such a torch if you do not have one.
Once heated cherry red, WALK AWAY until the next day. Let it cool naturally. The part expands as it is differentially heated from the block and squishes the fine rust, and then when the part cools there is a microscopic void between the parts.
Next, using a 6-pt wrench or socket, actually try to tighten the part first. Notice I said "try" to tighten. You will not actually be tightening anything. With some tightening torque you will actually feel the part 'give way' ie you will feel and often hear a slight sound as you break the rust bond. If the part has rotated more than 1/32 of a turn you have tried too hard. Once it 'lets go', then loosen very slowly. The yankee mantra is 'tighten first'. Go back and forth: loosen some, re-tighten some. Once loosed about a quarter turn now is the time to spray your fav penetrant in there. Keep going back and forth until it is out. If you encounter resistance doing this and the part doesn't want to move DO NOT FORCE it. This is about physics, not brute strength. Repeat the entire process by re-applying heat to cherry red, etc.
Propane etc doesn't work because not enough heat can be applied quickly enough and instead everything heats up so the 'squishing effect' is lost.
I have never had this method fail, ever. In fact I just removed the head from a 1932 GMC 1.5 ton truck that had never been off using these exact methods. All the bolts went into the water jacket creating a horrible rusted mess.
When you install studs in the block, some use Permatex #2 on the threads to keep water from creeping up the studs. That works or you can use another Permatex product in a tube: they make a Teflon paste that you use the same way as Perm. #2. Be sure your threads are chased and clean. This stuff makes a great sealer. Once the stud is screwed in wipe the excess.
Do yourself a favor and get new studs.