It could have an overfinch rebuilt 4.6 but as stated it should be badged. If it has a proper overfinch conversion it will be fitted with a small block chevy and should have corvette flags on the rocker covers. The chevy lump also looks completely different from the rover unit.
One sure fire way to know is to fill it with fuel and wonder at how the hell a full tank has only taken you 220 miles, or drive down an unlit road at night and enjoy the firework display in your rear view mirror as she spits flames on the over run, or look at the expression on the face of the sport supercharged driver who's just had his doors blown off by an 11 year old P38.
Great forum BTW.