Perhaps this will help. Regular steel and aluminum are unlikely to corrode together. Anodized aluminum will also keep any galvanic corrosion (GC) at bay. If you were to just put in an anodized aluminum fitting between the steel and aluminum, this would isolate the two materials preventing any GC. Grounding the aluminum radiator will also help. Anodizing is recommended for automobiles that use aluminum fueling parts and ethanol, since ethanol eats aluminum like kids eat Cheetos.