Neglecting for the moment the question of size and length availability. I've only been able to find 14mm in 18-8, which is not hardened but generally rated at 100 ksi. That's probably too low.
These are 14mm bolts or 0.551 inch in diameter. Cross sectional area is 0.238 sq-in, so selecting a 17-4 like H 1150 gives YS of 150 ksi. Something like a single shear of 35,700 lbf, so using a FS of 0.2 (5 times factor of safety) is still 7,140 lbf or 14,280 lbf in double shear. Although if memory serves something like this would not be a double shear joint due to the relatively large void between the side plates and the twisting of the spring in the hanger.