<legal hat>
Note, if you do this, what you are left with is a modified Range Rover and should be taxed, insured and registered as such.
from my interpretation of the rules you aren't going to "radically" alter the vehicle by changing the body and replacing engine and gearbox, however your interpretation may vary, especially if you do anything to the chassis. At worst it would need an SVA and a Q plate.
</legal hat>