I have pondered this.  I looked at the specs for the 45-70 and the 30-40.  Let's say both belts are designed for a snug but not tight fight for the respective cases and the rims are used to prevent the cases from falling through the loops. The base diameter for the 45-70 is .505 inch.  The Krag round is base diameter is .457 inch and the rim diameter is .545 inch.

So I am thinking the Krag round will be loosy-goosy in the belt loops but will not slip through the bottom of the loop.  It may pop out of the tops though.

This is all a guess.  It could be there is enough friction to hold the cases in place regardless of activity such as shooting from a trench and a mad dash over shell craters against Spanish machine guns.