I have a set of alumimun heads that have not been installed yet, I would like to paint the block without the heads attached is there any problem spraying a some paint on the deck of the block to cover under the head. What is the best way the paint the deck area where the heads meets the block.

Mask off where the heads contact the block . DO NOT paint the block surface where the gasket and head seat as this can cause all kinds of problems with the torque and seal of the heads on the engine !