Looks like you still have a chance on that one! was it tack welded and did you grind off the weld first?
Id heat the piss out of it and clamp on with a vise grips. Or weld a new bolt to that one and then heat the piss out of it. Either way i guarantee with some heat it will come out!
As far a bolt size im gona guess a 3/8" fine thread. Grade 5 would be fine, but i like the grade 8 on alot of stuff because there gold coating makes them more resistant to rust. A coating of antisieze should help from hapining again.
Dont give up so soon. Heat works ausome on things like that. GOOD LUCK!