Okay, here's my dilemma...

I have attached a picture of the base that my friend has just had installed on the shoreline of his cottage. The structure will be the base for a platform that we will be attaching 5/4 deck boards to make into a platform. Once that is done, there will be ramps and floating finger docks attchaed to it. The base is a solid structure in that it is constructed of 5/8" C-Channel steel beams. The posts for the frame are 12' posts driven into the ground with a backhoe that was brought in on a barge.

Anyway, the problem is in how do we go about attaching 2"x4"'s (for nailers) to the steel joists. We will be using the "nailers" to screw the deck boards into as well as putting 2"x12" boards around the perimeter of the frame to act as nailers. We will have to drill and bolt the outside frame to the outside C-Channel beams. The only two options we can come up with for the nailers is: 1. bolt the nailers to the C-Channel brackets by first drilling holes through the steel and then bolting them to the steel or 2. using some sort of galvanized strapping and tying them down to the steel joists. So, I was hoping to get any recommendations as to our 2 ideas, that or anything else that anyone can recommend. What type of drill bit should we use to go through the 5/8" steel?

Also, we have scheduled this Thursday as the day to do this - tight schedules for both of us... so, I would really, really appreciate ideas as soon as possible.

I have attached a photo for reference...

Thanks a ton in advance!!!