I'm a big fan of the torpedo hinges, but as stated above, you do need to shim them for clearance. We made our spacers out of aluminum plate, and I too was curious about the compatibility of the dissimilar metals. (My trailer is aluminum skinned also.)
The hinges are made of zinc diecast w/chrome plating. The chrome plated hinge has an anodic index of 0.60, where the aluminum has an anodic index of .75. This difference of 0.15 is right at the edge of acceptable compatibility requirements for harsh environments (outdoors, high humidity, salt).
http://www.engineersedge.com/galvanic_capatability.htm
I suspect that hinges and aluminum spacers will last a long time and stay strong.
Gern