years ago, I put 2 x 10's on my 16' trailer. I drilled through the wood and rails with the tap minor sized drill, then drilled the same size hole (as the bolt) just through the wood, then I tapped them with a pulley tap (long version). After screwing the bolts into place, I still got underneath and put on some poly lock nuts.
Oh, I put the first 2 boards on by bowing them with a jack and big tractor FEL, then lowered the jack and let them slowly into place, but that was quite dangerous with the 10" wide boards, so I just cut off the angle iron on the back edge and then it was eazy peazy... Just wish I had a tig back then to weld the angle iron back into place. I used a stick and got the wood hot enough to smoke at every weld...