The VB Commodore made from 1978 till 1980 came with 4.2 or 5.0 litre engines, meeting the requirements of the day in terms emissions etc.
A whopping 87kW and 114kW respectively.
VB Commodore Specifications
Cast iron technology is old technology, however strong it may be. I don't dismiss it as an option because of that, rather on the terms of mass and how such an increase will effect the dynamics of the LR into which it is fitted.
You could always just buy a later Range Rover with the Jaguar V8 engine. I recall they were in the order of 500HP, totally modern and not at all agricultural.
Ron.