Taken from the DVLA site, more reliable than 3rd party sites:
A rebuilt vehicle can keep its original registration number if you can prove you’ve used:
the original unmodified chassis or bodyshell (car or light van)
a new chassis or monocoque bodyshell of the same specification as the original (car or light van)
You must also have 2 other major components from the original vehicle from the following list.
suspension (front and back)
steering assembly
axles (both)
transmission
engine
Get a Q registration number
DVLA will give your vehicle a ‘Q’ prefix registration number if you don’t meet the conditions for keeping the original registration number.
Your vehicle must pass the relevant type approval test to get a Q registration number.
https://www.gov.uk/vehicle-registration/rebuilt-vehicles
So if the chassis from the Range Rover is unmodified, and is still using the Rangie axles, suspension, engine etc then it should be registered as the original Range Rover. If the chassis has been shortened, or the vehicle is using the Series Landy registration then check the V5 for date of first registration, this is not the year of manufacture date, but the registration date. If it's passed an IVA test the date of this test will show as the registration date.
The DVLA test center will issue an Individual Approval Certificate (IAC) if it passes. You’ll need this certificate when you register your vehicle. This registration date will then show on the V5.
With the above info and looking at the V5 registration date remember that SVA tests came into play in 1998, and were replaced by IVA tests in I think 2011. So if it's had a test the registration date should be after 1998 I believe.