Well the one thing you didn't mention was how much you are willing to spend. And with out all the specs of the engine it is hard to tell.
You could always go with a bigger stoke. 4.500" will give you 598ci and a 4.750" will give you 632ci. And we all know that more CI will get you what you want, more power and torque.
just my 2ยข worth
gyrogearloose