Questions: self drilling/ tapping screws for chassis work

I'm getting ready to install my underbelly cover -- there will be aluminum C channel beams (to support the foam panels and the coroplast under-skin) attached to the underside of the trailer's frame members. Some of the steel frame is pretty beefy -- either I beams or box beams.

I know some folks have used beam clamps to attach the C channel to the chassis, but I am very risk averse and want it all to be screwed down in a bomb-proof way.

I thought about quarter inch bolts -- yes, that would work, but that would entail a lot of time drilling up into the frame while laying on my back with bits of metal falling in my face. Sounds like fun! Plus it involves drilling big holes into the frame, potentially weakening the beams.

So I am leaning toward self drilling self tapping screws. I am planning to drill a pilot hole -- I will experiment to see what size pilot will give me the best results.

I am leaning toward hex head screws -- I have a tendency to strip Phillips screws when working upside down, and hex heads give me a nice solid grip, so that I can snap the head off a few of the screws by over-tightening them. (I wish I were kidding -- I have a terrible problem of knowing when to stop tightening things. My wife calls me "The Great Twisto" in honor of the things I have destroyed.)

But here come the questions -- first, is there a particular brand of screws that you prefer?

Do you use an impact driver to install the screws, or a drill motor with a socket?

In thin metal (let's say a sixteenth), do you need to drill a pilot hole?

Thanks in advance for your advice. I am guessing that there are quite a few of us who are not experts in using this type of screw, so your comments will be appreciated!