drilling into metal is not so easy

I'm working on attaching the trailer flashing to the trailer. I got sheets of 4'x10' galvanized steel off c-list, really good price. Hoping that they will be a thick enough gauge. I had to pressure wash them first as they were quite filthy. Before:

 And after:

To begin with I have to attach something to bring the level of the joists down to 5 inches (they are only 2 1/2). So I'm attaching 6' lengths of 2x6s that I've ripped to 5". Here's what it looks like:
Next up is puzzle piecing the metal sheets together so that they overlap properly and all end up resting on joists.
Onward!