I think that £8k is conservative, especially as it reads as though you are looking for a drive in drive out conversion. Seeing as you want to push for 400bhp, i wouldn't recomend a Rover V8, it WILL cost you ££££'s on the engine alone. And hypothetically, if you want the car to be reliable with that kind of power, you'll need to uprated most of the other components too, the torque of V8's is a killer.
I don't mean to put a dampener on things, it just pays in the long run to be realistic. I run a Rover V8 in my everyday car, have for over 5 years now. It would be great to give it more power, but i don't have the time, space or money to play with it. Any how, as it is, it's RELIABLE , and that to me is pretty damn important. I've chosen to build another car, at my leisure and pleasure, and it will be ready, when it's ready, even if that's later than i would like.
If you make it to the 'Horse and Barge' next week, have a chat with 'tiny', i know his type of build isn't what your looking for, but it will give you an idea (especially on build costs). I might be there too?