If you are necking the 30/40 case to 35 caliber you are making a 1/10 in short 35 Winchester case.  Not sure how much 35 Winchester data is still around (let alone pressure tested) but you can use the starting load data as a guide.  The 35 Win was used in the 1895 Winchester lever and had a working pressure of about 45K psi vice the 40K psi for a Krag.  The case capacity comparison given for 358 Win and the 30/40 makes them seem very similar, but the data I have seen is based on the case filled to the mouth, meaning the actual powder space in the Krag is a lot less than in the 358 and probably a lot closer to 35 Rem.  The ballistic of the 35 Win, at a slightly higher pressure level are only slightly better than the 35 Rem. Before betting any body parts it is good to remember that even the newest Krag action is over 110 years old.