If I were in your boots I would use spacers.
Use bolts and spacers (Al or Cu tubing as sleeve) between rad and fan. The gap could be sealed off using hot glue gun (glue sticks appropriately cut to size and glued, using some LEDs in between would make it look cool) or sponge nicely cut or packing material.
Look no futher on 'how to make shroud', we do have talnented people here:
http://www.procooling.com/articles/h...ite_air_du.php
(By #Rotor)
Then again if you did find a nice shroud, buy it and use it.
For mixing of metals, till date I have not witnessed galvanic corrosion myself. It is possible but in long term. Using a high coolant to water ratio and maintaining the rig voltage to zero (by properly grounding) would slow down the process. In any case, it would take years.