Here is an extract from ARonline about the Rover 75, it doesn't seem the 75 was ever made to have RWD.
"It's a BMW in drag"
The Rover 75 was the first product of the BMW/Rover alliance to hit the market, and was a giant step forward from the Rover 800. However, many people believe that the car's chassis is based upon the BMW 3 or 5-Series. This is certainly not the case, and its large transmission tunnel was set-up in order to give the car impressive torsional rigidity. There is no space for a rear differential without significant modification, as MG Rover/Prodrive has found during the development of the ZT V8. The basis of this story lies with the fact that during the months following the BMW takeover, a concept called "Flagship" was developed, as styled by Richard Woolley. It is Woolley himself that explains the situation: "The story originated from the fact that very early on during BMW ownership, we did look at 're-cycling' the then outgoing 5 -Series platform for Flagship. BMW were about to launch the new (current) car, and all the tooling for the old model's underpinnings were theoretically available, sourced from the South African BMW plant. It was an idea that BMW suggested we investigate." This large car (a kind of latter-day P5) would have sat on a modified BMW 5-Series platform, but was cancelled shortly after the styling proposal was completed
