A Lefever Nitro Special, if in mechanically sound condition, can be safely used with any modern ammo it's chambered for - in a 12ga, that's 2 3/4".
FWIW, all SxS doubles should be checked for a tight/loose hinge via removing the forend, turning the shotgun upside-down, and trying to wriggle the barrels while the opening lever is held in the open position.
They should also be checked for being on/off "face", via viewing the joint between the action face and the rear of the barrels, from the side, with the gun/barrels "closed", and a strong light behind the gun.
Any opening, or angle should be evident.
Never dry fire them - changing FP's is a PITA.
OH ! And.......never pull both triggers at once.
( Don't ask