Spinning a mill to 8k with 425hp is easily doable. Not cheap for sure but many good engine builders will be able to deliver that.
If you want to go a little further ....... give these guys a call
http://www.coatesengine.com/index.html
With their heads on a strong bottom end you could spin it north of 10k. The fuel system would need careful design to keep up - but again any good builder will be able to do that. It would sound .... interesting.