From here, first source I found withing 10 seconds of looking:
http://www.6r4.net/site/index.php?option=com_content&view=article&id=3&Itemid=4
Having rejected the Honda V6 (Austin Rover then shared a great deal of engineering development with the Japanese manufacturer), the only other obvious solution would be their own tried & tested 3.5 litre Rover (formerly Buick) V8 engine. Being that bit too bulky to fit in the back of the Metro, the decision was taken to cut out two cylinders and (literally!) weld it back together - hey presto, a 2.5 litre Rover V6! Fitted with a purpose made crank, Rover Vitesse racing internals and Weber carburettors it produced a healthy 250bhp - ideal to test the car until the final engine was ready.
I have read the rest, as I was going from memory as I already said. Once again, going from memory, I think the 250bhp models were the club models, and the 400+ bhp were the works versions.
Pretty sure they made a purpose made block soon after.
I laughed at the response I got, as some ways he's right

Why should I stick my nose in when I see something I think may be misleading/dishonest.