Ya gotta put 'em to the frame only. And when you mount 'em, make sure you leave at least 1/2" between the slider and the closest point on the body, because your frame will twist that far if you put the weight of the truck on the slider...been there...done that.
FWIW, I'd suggest a gusset between the extensions from the frame to the slider. I've done quite a few sets of sliders so far, and found it made a significant difference in how much the sliders flexed relative to the body.
When you go to install them, you will find that you probably will have to drop the gas tank to bolt up the driver's side slider. There just isn't enough room to bolt it up without moving the tank.