wow. ummm for one, a 72" rotary cutter is every bit of 6½' wide, and nearly 9½' long.no I'm not kidding. I've carried a 6' bushog in the 6.5' box of my F150 without any problems. Yes ,one side is up against the wheelwell/bed and it's resting on the side lip of the box, never been an issue.
Have to ask why do you think this is 'wrong' ?
The inside of a pickup bed is ~65" wide and usually 6½' or 7½' long, or maybe you can open the tailgate, good luck towing like that. Once you have "tossed" this 750 pound brush hog in the back of the truck, how do you plan on getting it out, and back in, at the job site without totally wrecking your vehicle? You want to lay the cutter on its edge and rest the other side on the aluminum bedside? sure
I have an F-250, there is no possible way even a 60" cutter will fit in that bed. Here is a 60" cutter sitting behind it. FAT CHANCE. Even if you loaded it as you suggest, it would have damaged the truck, taillight, or body, due to the geometry of the cutter. Certainly would not have stayed safely in the bed.
To give you further perspective how delusional your "advice" is, here is a photo of a 48" cutter in my F-250, it fits by only a few inches, and you are suggesting a 72" to "just toss it in the box" gmab