range rover is not as long in the nose as a discovery, it does leave a gap on a range rover between the bumper and front of car if its a disco one.
but they do mount the same etc.
as the disco front is all plastic, if its a proper classic bumper, it should squash onto the front of a disco, but will be tight up against the body, and not sure how it will sit if it is one that goes around the wings at the ends.
if its the right price make it fit and chop about to how you want it to look.