Frozen head Bolts

[BRIELLY]

I'm attempting to replace the head gaskets on a 96 yamaha 250. I have 2 head bolts that don't want to come out. I assume if I break them off in the block I'm looking at pulling the powerhead and having it drilled out by a machine shop. Most likely I'll run into more broken bolts trying to do that. So whats the best meathod for removal? I was thinkning of heating the bolts with a propane torch and trying again. Would an air impact wrench be good or would that break the bolts? I'm pretty sure penetrating oil will not make it into the block to help loosesn the bolt. I was also thinking about breaking the heads off the bolts off so I could at least get the cylinder head off and have a chance at getting the penetrating oil into the area where its seized. I could use channel locks or even weld a nut to the remaining stud at that point and have at it again. Any thoughts or suggestions would be greatly appreciated.