im not sure that the studs are left hand thread, the best way, if you can get a dremel in there, try to dremel it out, a drill bit can walk a hair, and when you start sizing up in bits a hair here, is way off there. since the dremel doesnt cut as fast as a drill bit would its a lot easier if you get off center a hair to correct it
when i have problems like that, ill grind the left over bolt flat so i can center punch it, then ill start with the dremel, ill make a round hole dead center with the dremel, then start with the smallest drill bit and work my way up, ill go up in bits untill the threads pretty much come out on there own.
then you can use a screw extractor or even a sharpened flat head screwdriver, hammer it in a bit and unscrew it