I'd be worried about this too.
No matter how you slice any of these plans, none are really good.
I think the real difference is that if you build (IMO buy) a 302 and put aluminum heads on it, even if the future never comes, you will still have a pretty kick ass setup in a light car that pretty much flies.
If you build a 347 and you never get around to the heads, it will always be a mediocre, disappointing, unfinished project.
The only solid plan would be to build the 347 from the ground up on a stand and finish it with the permanent parts, not temporary ones.