The N3 would be the higher one.  You may have to use the higher one for bolt clearance.  The old list of the Q, U, N, and B mounts that I have doesn't have the Krag on it, however.  It's a copy someone gave me out of an old Shooter's Bible.  It seems to me there were some N mounts made that take a 7/8" tube like the Lyman Alaskan and several others.  The Alaskan is a bit pricey now a days, however. 

Found a pic of a list online, N2 and N3 are the right ones for the Krag, with N3 being the higher one.

They are a solid mount, but wheter or not you have enough adjustment on your scope will likely depend on how good a job the machinist did getting the holes parallel to the bore line.  A lot of 3/4 inch scopes are short on eye relief for shooting highpower rifles as they are meant for the low recoil of the 22LR.  For example the Realist 3/4" scopes only had a 2" eye relief.  Probably wouldn't be much of a problem with the recoil of a Krag as long as one has a good hold on the rifle.  Weavers have a more generous eye relief of 3-3.5"