Zero, you are facing a prob that drove me nuts until I found a half-way acceptable solution.
My first prob was, as yo mention, those darn ribs. The second was- how to mount anything with any security? There's not much to the sheet metal.
My best solution so far is to mount a pair of pieces of strut to bridge over the ribs. I attach the fixture to these with strut-nuts. The strut is anchored using toggle bolts- nothing else has any strength.
Where the box, or mud ring, exits the sheet metal I put a "bell box" blank cover. In that cover I have already mounted a connector for a short piece (maybe a foot) of liquid-tite. It is important that there be enough extra so that you will be able to access the box easily, without moving the light. In the liquid-tite I run the hi-temp (appliance wire) specified for the light. Wiring inside the building can be your standard THHN.
As a side note, I have gotten better results by having each light have its' own photocell. These in turn have worked best when aimed straight up.
