Thanks for reminding me about the missing pics on mine. I did some Photobucket reorganizing and completely forgot about fixing the links.
They're fixed now, at
viewtopic.php?f=22&t=1960.
To the original question - yes, they are intended to mount directly to the trailer frame tube. They are indeed some big holes (1/2") and those tubes are a biotch to drill through. If I were to do it over again, I would consider just mounting them directly (per the instructions) but after having them for 5 seasons, I'm still glad I did it the way I did.
Walt, I don't remember ever seeing (or knowing) that you mounted yours on the stanchions. I like that idea too. Like the OP, there's just something I don't like about drilling into the tubes.
Finally, I would add this installation tip - coat the threads of the mounting bolts with anti-seize
before you install the nuts. If for any reason you have to back the nuts off and you don't have the threads coated, the threads will gall and jam almost immediately. You'll have to cut them off and then find replacement hardware.