Headlight, I know it looks tight in there. Thats because it is.
Edited after re-reading your post: Mine does hang down slightly below the bottom of the bed, but not below the frame.
I ended up using the supplied bracket, and drilling & mounting it to the vertical cab-support bracket on the frame.
The hard part was holding that heavy beast up in position w/ one hand, and using an automatic center punch to start a drill hole w/ the other. Also used a small pilot drill to make the starter holes, and drilled them out to size on the next step or two. Use fresh, sharp drills, and turn them slow, so you dont burn them up making them soft.
You can also tuck it up pretty high under the bed so that it doesnt hang down below the frame. On mine, the filters just barely hang down below the bottom of the bed, but its really not bad. Cant see it unless you get down there & look. We also do alot of off-roading, but it shouldnt be a problem there. About the only thing that could really touch it, is maybe a tall bush, but by the time it got to the filter, it had to do alllot of scraping on the rest of the truck first. Not too bad really. If I get a chance will take a pic and post it.