I agree with Mike P, if the car runs fine now leave it until you have the money and parts to do what you want with it. Why spend more money building your V6 when you really want a V8. by doing one then the other you are just wasting money that would go into putting the 390 in. Makes sense doesn't it? Plus, once you have finished the V6 you would most likely not be wanting to do an entire engine rebuild anytime soon after, again.

That's how i look at it.