Those are good points!
I chose 4340 because even in it's annealed state, it's quite stiff. Heck, even at forging temps, it's quite stiff.
As for the information, I'm going on something like my 275th pound of thermite over the years. Lots of trial and almost as much error.;) An occasional success, here, and there. I do , finally, have the refractory down though. Took awhile....