Not without a lot of work that would cost a fortune, it would probably literally be cheaper to just buy an S-TYPE R in the first place ?
To do it right, you would need a full car as a donor anyway, not just the engine.
The 4.0 V8 is the first generation pre-facelift model, the STR is only available in the post facelift version, so there are a lot of other differences other than the engine.
You would need the brakes, suspension and gearbox swapped as a minimum, you would then need an engineers report for the car to be insurable and legal on the road.