No reason why you can't.
Dad's 90 is a 3.9 v8 using a TD5 tank.
You will need to buy the tank and fuel pump and find a way of patching over the current fuel filler then moving it to behind the rear wheel to suit the TD5 tank.
The only problem we had was that the submersible pump is usually lubricated by the diesel, however petrol isn't as thick and caused the pump to seize.
If you already have the parts, then it may be easier to stick with the current position.