The combined growth of an aluminum block and aluminum heads at temperature is just about enough to change the valve lash from zero to .018"; in fact I will lash my valves on all aluminum wedge FE motors at zero, or just a tad looser, when they are cold, and then at hot they are around .018". If you figure a 1.75:1 rocker ratio, that means that the head and block together are increasing in height about .010". Of that, I'd guess that the block is about .006".
So, if you were to keep the revs down until the engine warmed up, you might be able to cheat the quench distance by .006". But I agree with Howie, .040" is a good number to go with, and the extra .006" you might be able to reduce it by with an aluminum block won't buy you too much, and poses certain risks. All the aluminum engines I've run have been set up with .040" for the quench distance.