A chain breaker should fit, but getting it far enough back for the rear chains may be hard, especially getting past the brake discs. I'd rather not go in there with a grinder, all the metal and abrasive particles from the wheel will get everywhere. But if there is no option, you may need the grinder. If cutting was the last option, i'd like to get in there with somethilg like a plasma cutter or oxy torch, even if you need to simply melt through a link with a brazing tip with an oxidising flame, that is if the cutting tip wouldn't fit.