Man, that's rough but honestly I would have started with the half inch drive every time. Those old Otis bolts are basically welded in place. My move is to soak it in penetrating oil overnight first, then hit it with the big gun and a long breaker bar. If that doesn't work, it's time for some gentle heat before you round anything off.
The half inch drive is a good start, but I've had those bolts laugh at a breaker bar. Last time I used a four foot pipe on the handle for extra force. You really need that long cheater bar to get enough torque. The overnight soak is key, but sometimes you need two full days of oil. If heat is next, keep the flame moving so you don't wreck the surrounding metal.
Have you ever tried using a torch on one of those bolts while it's still soaking in oil? I did it once on an old tractor and nearly burned the whole thing down. Learned the hard way that fire and oil don't mix, so now I just stick to the pipe and a lot of patience. My neighbor swears by a propane heater but he's always got a fire extinguisher handy, which seems smart. Anyway, that half inch drive is fine but I've broken two of them before I got wise and bought an impact wrench.