Another consideration that is often overlooked is the costs involved with re-working the rover drive unit to handle extra loads. I've read about the up-rated input shafts and output flanges, but can't imagine them to be cheap, then the weak point is transferred to the pinion gears in the differential as harveyp6 says, leading to potentially more costs and uncertainty to the reliability of the drive unit as a whole.
As Sharonandmatt have already indicated they have a possible source of axle narrowing services, then the live axle is definately a route worth following if at all possible. I've been doing some research and found that the original factory p6 racer used a jaguar differential and the rest was a reinforced rover de dion setup, so if you're intending on keeping everything as close to period correct, then either way the jaguar route is worthy of consideration.
Another point of consideration is that if the top trailing links are relocated to inside the line of the chassis legs, then you will have more room for wider tyres in the rear without the need for tricky body mods. Utilising a live rear axle, you will not have the problem of the track changing while the suspension works, so allowing a better axle/wheel/tyre combo being chosen that will only move vertically as such, and not in addition latterally with the de dion axle (if that makes sense).