I have removed and installed many toilets. When they are corroded I simply use small electric cut off tool. Dremal is a common brand but for this work Black and Decker makes one that only costs $20-25. Like it better. Then buy a metal cut off blade (1/12" or larger diameter works well about $3 for two) and buy the heavy duty arbor ($3 the blades work better then with the standard arbor). The tool is small enough to get around into narrow spots. Then simply replace the bolts with new ones.