Been there, done that. Was much easier than I thought. Just let the other half take the stock one for an extended drive, and tell her not to worry about the fuel bill. Mine was hooked and still hasn't given it back.
Once hooked, tell her how much sense it makes from a sparing perspective that you both drive P38's so can swap parts when troubleshooting. When you buy random spare parts and go-better bits, it could be for either car so you are covered... you are experimenting on yours for safety and reliability before transferring to hers.
I've watched my better half talking to friends and new acquaintances... she loves to tell them how she drives a Range Rover... Sometimes she even tries to justify it by saying that her husband does most of the servicing :)
There are so many reasons your better half should be driving a Range Rover (ignore the fuel bill)... her safety, your kid's safety, your peace of mind, better/higher driving position, handling on-road and on rural tracks and wet conditions.
Is parking a problem? Install a reversing camera and small screen near the centre air vents... no longer a problem.
Next? :)

