I agree completely with Andy's comments. If you wish to go down the route of a large capacity, T5 gearbox Plus Eight, you are starting with the wrong car.
The chassis is narrower and different from the 5 speed Plus Eights, the track is narrower and the wheels narrower also.
If you have bought a one owner, Moss boxed car and wish to effect the changes you describe, you will destroy the value. A later car will provide a better base to start from.
However, it's your car and your choice.
