If you do the common track change thing to the front of the sprinter (with the sigma control arms etc) then a 20mm increase on the rear would be a good thing imo.

So for an RA40/60 to fit the panhard rod needs to be changed, no big deal. What about the trailing arm mounts? Are these in the same position? (in relation to the diff centre).

Just to add my experience: I picked up a t18 rear end (one without sway bar) and an ra40 4.1 centre from my local u-pull about 6 months ago for something like $70, there was only 1 t18 at this wrecker but there were probably about 10 - 15 RA40/60s. Sway bar mounts were $40 to be welded on and axle bearings were another 40 so all up it is not an expensive exercise. The only 'upgrade' really is the 4.1 gearset, another $1000 aprox would be needed to get an LSD (or another ~$50 for welding the current centre)