Case hardening steels are by their nature, a compromise. There are stronger steels available better able to resist stretch, which is the limiting factor in the postwar crank for gripping the crank pin. Even the factory did not trust their cranks for more than one pressing. Factory reconditioned units were honed +0.004" to make the hole nice and circular again and oversized crank pins made. They were then etched +004.
Now I do not know if the postwar crank webs and throws were case hardened; never checked. I do know the prewar one were not particularly hard, certainly not over and above the core hardness heat treatment, so at that time they were not too worried about the side of the conrod eye boring into the face of the web. This is with the built up cranks, which generally had a separate crank pin race. Where the crank was in one piece, like the DT, the face of the center web was case hardened at the same time the crank pin surface was. The opposing surface on the removable counterweight was not case hardened. Mind you, if the cage (where used) was skewed, it bored into the center web no matter what. Never the easily replaceable counterweight, oh not never!
So if case hardening is not needed, one of the through-hardening steels would give you a much greater tensile strength. A G50 and a Manx bottom end has a lot more metal around the crank pin to resist stretch. I am not sure why they would use a case hardening steel for that as don't they have removable hardened thrust washers anyway? It might be because that material it is readily available in the large disks needed. Sometime is it just marketing. "Made out of EN36", which any vintage motorcyclist is likely to have heard of at one time or another. They don't use the BS 970; 1983 designation, do they?
The only thing I can think of that having a high surface hardness will aid (beside resisting side wear from the conrod) is resisting galling when pressing up. And surface finish has a large part to play in that too. If so, they you only need a very thin surface hardness. In that situation, nitriding is an advantage. You get a hard surface, no distortion, and no alteration to the core properties. And you can use it on through hardening steels that you cannot otherwise case hardened, like SAE 4340 (EN24). The core strength of 4340 is way above EN36. And if 4340 is not strong enough you can alway use 300m (good luck affording it!) 4340 and nitriding is being used here for a lot of custom and short production run gears, supplanting the more traditional use of 8620 or 9310 with case hardening. Though, if you need a deep case for maximum wear or dynamic surface rigidity (heavy tooth contact loading or for bearing rollers), then a through hardening steel with a superficial skin hardness from nitriding is not sufficient. But a press up joint is a static load.
Just some food for thought.
-Doug
