You'd have to pull the oil pan to truly confirm if it's a 4.6. Your serial number doesn't seem to align with any Rover engine according to the data I've found doing searches. If you say it has plenty of power then why do you care? I'd look for signs that there has been front end damage with a chance the radiator support has been replaced.
https://www.v8register.net/FilesV8/V...2030.11.07.pdf