Legally-speaking you'd be better off running a blue block so the pollution gear requirements match what's fitted to the car, and then you're free the change the head/manifold/fuel injection/whatever to a later model setup without the need to comply with the later emissions standards (since the definition of "engine fitted" is the block being used).
It probably wouldn't matter as most "inspectors" couldn't tell the difference between a blue motor and something else painted blue anyway, but you never know how diligent an insurance company may be years later whilst trying to wriggle out of paying a claim!
Andrew
