Personally, I'd go 75mm regardless of your future mods. Later model SOHC engines aren't plagued by low end torque and horsepower loss accompanied by large throttle bodies like the old 5.0L's were. Going up 10mm in throttle body diameter isn't gonna hurt anything. It's true, that you may not see any improvement over a 70mm unit right now, but if later on down the road you do decide to move some air through that mill, it'll be nice to know you don't need to upgrade that particular component again. Not to mention they’re usually had for the same or very little difference in price.
Your choice really.