If you want the ultimate v8 for a seven it has to be the tvr melling ajp engine.It is 10 kg lighter than a zetec anything between 375/450 hp depending on the state of tune, comes complete with throttle bodies, it is the smallest lightest v8 I have ever come across. Can be picked up for as little as 2k with a t5 gearbox. Fits in the engine bay with ease < no bumps in the bodywork needed, I recently missed one on ebay complete with gearbox and ecu for £1750. You could spend more than that tuning a rover lump.
http://en.wikipedia.org/wiki/Melling_V8
http://www.pistonheads.com/pics/news/19361/ajp82-L.jpg
Bob