300 axles will physically fit wether on a 90 or 110, however discovery 300 rear axles come with a three pronged flange on the differential, this needs changing to a four bolt one, v8 discovery did not have a 4pin diff.
The only thing is, that it's claimed that the standard rover axle isn't rated for the rear of a 110, not all 110's were fitted with Salisbury rear axle, so they must have done something to the rover axle to allow it to be fitted on the back of the 110.... No idea what though!