When I had my axles out to replace seals, I recall that the retainer bolt for the spider shaft had a short threaded section at the head and then a smooth pin. If it broke in the threaded portion, you might try a left hand drill bit to get the rest of the threads and pin out. If it broke in the smooth section, you may be SOL especially if the shaft has turned with the retainer pin still in it. But there's a possibility you could use a strong magnet tool and extract the pin if it's still lined up with the hole. If you can drive the spider shaft in a bit, then maybe the pin will come out with the shaft.
I don't think the pin is hardened, as it really doesn't have any load on it. The overall length should be from the spot where the head sits to just short of the opposite side of the spider shaft.
Good luck.
