I would recommend something with both high strength and high fracture toughness like SAE 4140 or SAE 4340 as a starting point. More highly alloyed steels like 300M (a modification to 4340 by adding silicon) and tool steels provide higher strength and wear resistance, but lower fracture toughness. Optimum properties can be produced by modifying the heat treatment (tempering time), but a first guess is that it should be heat treated to a hardness of ~ 35-39 HRC.
Wow! You play with big toys. Does a 8 inch nut need about a 40 lb sledge for proper torque? I have also seen similiar set-ups where the wrench was tightened and held with an overhead crane while striking. If this method is used, leave the wrench long enough. Wait a minute, the overhead crane will be needed to move the wrench.
I would investigate the proper welding on the material before purchasing. That way mistakes can be repaired, and the design features that only becone obvious at application can be added.
