Either raise the front, or lower the rear. Wire sheath is a cheap and quick alternative to slide over the front to make it taller. You could file the rear sight base where the slider sits, or file the slider itself. You'd have to look closely to see where it contacts. Could be it may not take much if there is a high spot or something.
I had a tape measure and a 91/30 handy, so I went to the link dave provided for the alteration calculator. From the rear sight to the middle of the hole at the top of the front sight, is 24.5 inches on the measurement I did. I used the example of the rifle shooting 6 inches high at 100 yds. which seems to be pretty normal for refurbed 91/30s when the rear sight is at its lowest setting.
Amount of error = 6
Sight radius = 24.5
Distance = 3600 (3600 inches in 100 yds)
After calculation, a result of .041 alteration would be required of removing material from the rear sight, or elevation of the front sight to get you on par @ 100 yds. according to the correction calculator.