Ed,
Thank you, I'm not familiar with 4335 but I'm thinking the 4340 is probably best for strength as well as impact/fracture resistance. The walls are 9/16" thick. I found an old ASM book on heat treatment and it looks like normalizing at 1600F, quenching from 1525-1575F and tempering 2 hours at 900F will give HRC41-43 as well as toughen it for impact.
Tug,
That's a good point but I don't think there's much that can be done about it. Probably part of why they don't make impact sockets this size - diminishing returns, along with weight, cost, demand, etc.
Thanks for the idea, Superbolts are interesting but this is for an existing design and they won't fit. Worst case, we go back to using the impact spanner and hammer, my main concern is this thing taking 3000 ft-lb torque...