Still haven't found the ad I referred to, but still looking.
Did find quite a bit on US sites about GM 6.2/6.5 into Series and Defender so RR shouldn't be impossible, but they all say leave earlier 6.2 alone and go for 6.5. There are also quite a few comments about the need to mod the drive train significantly to cope with the increased torque, particularly about it eating LR auto boxes.
They also say they hear it's a common conversion in the UK, if so where's that damm ad? It may be a case of finding a rusted out donor car to get the right bits.