Jack,
As nitriding is pretty thin you may be ahead to pre-grind the areas that you plan to machine to remove the nitriding. This still doesn't do anything for you in the 42Rc 4340. Nothing kills tools faster than not having a stiff set up and looking at your mill set up I think I might try to move the head down as close to the crank as possible and then pull the quill/spindle back up as short as possible and also maybe consider using a small, 3-4 inch dia face mill with indexable carbide inserts and couple it as close as you can to your spindle. Turn it slow and lots of coolant. I worked on a project where we were cutting some 4340 that was around 35-40 Rc and we were using a 12 dia indexable face cutter, 6 inch spindle, and it ran at around 75 hp to make the cut, 12 inchs wide, .2 to .3 inches deep, probably not more than 50 rpm, don't remember the feed speed but the operator had to stand over the chip pile with a fire extinguisher to keep putting out the small fires from the slight amount of spindle oil and coolant that keep starting to burn. The chips came of the part with a very nice red glow! Just a note, the spindle was actually a 50 hp spindle but we were doing a 12 hour, 150% test to prove out the spindle. The machine was a Giddings and Lewis 6 inch spindle floor mill and we had to run this test with the spindle only extended a couple of inches any more and the cutting tool would not live.
Rex