Anything is possible if you have enough money to spend. Given that the engine ECU and Gearbox ECU work together it is likely even if you can get the gearbox to physically fit you would need someone to re-program the gearbox ECU as the V8 diesel ECU would not work due to being set up to work with different torque and power perameters
First problem, TD6 gearbox has five gears...TDV8 box has more than five gears....between six and eight depending on the year.
Need I say more?