I took 10 inches out of mine. I cut the frame right behind the double shackle for the chevy spring. You prolly arn't running chevies so you can cut a couple more inches offthe frame. and the back of my bed sits inline with the back of my shackles. You could actually make the back of the bed inline the the edge of the frame. It would take more out of the bed, but like you said, you don't know were your shackle will sit with the suspension.
