Galvanic corrosion, which is what you were inquiring about (contact of different metals) requires contact between the two metals. So a space between them does prevent galvanic corrosion.
Old dog is referring to electrolysis, which is the flow of electricity between metals and fluids. Its the same process used to do plating. This is bad news as it can create leaks in your radiator and other items. Search this board for electrolysis and you will find a few threads on that topic. Here is an example -
Zinc Anodes for your cooling system