My suggestions mimic those already given, first the penetraing fluids, and if that doesn't work, try heat. Cut them out as a last resort. At any rate, once they are removed I would replace them with a good grade/quality bolt, stay away from cheap hardware, it will only cause you grief down the road (and probably at the most inconvienent time).
1997 25' Safari
2012 Chevy Silverado HD 2500