You can modify the battery tray if you wish BUT be prepared to have to move other items around if you do, as there is a domino effect.
The Rover V8 with 9.35:1 compression ratio fitted to a 1999 +8 is not a difficult engine to start. It requires a bit longer cranking than some, but should fire up easily and positively if all the settings are correct.
I think it is more important to keep the battery in tip top condition than worry about the ultimate cranking power. I use a battery conditioner whenever the car is in the garage, and I do inspect it whenever I do any routine maintenance at the rear end and have the parcel shelf removed.
