The AR25 won't last long on the 3.0. The correct transmission for the 3.0L is AR35. The numbers 25 and 35 imply the maximum torque the box can handle, i.e. 250NM and 350NM respectively, which is achieved by having more clutches within the gearbox. The AR25 will be subject to premature heavy wear when used with the high-torque 3.0L engine producing 260NM which is over the AR25 box' rated maximum of 250NM.

I would expect that the ECU would be the same for the AR25 and the AR35 - but I don't know this for fact. Can anyone advise?

The AR25 was fitted to the 2.0, The 2.5V6 and the early 2.5 TD.

On the V6 it had more clutch bands to deal with the extra torque.

On the Deisel, it was just the same as the 2.0, but the amount of low end torque was ripping the boxes apart, so Vx started fitting the AR35.

The AR35 has even more clutch bands and was fitted to the 3.0.

I would start looking for an AR35 from a 3.0 (Should be less that £100 sheets from a breaker) while the AR25 is still going.