I have a '75 C20 Longbed Fleetside. I'm a 15 year old who is working on a project, and need any help I can get. My plan is to shorten the frame and make it a shortbed pickup with a stepside bed... From what I have heard you need to take 14 inches behind the cab and 6 inches off of the rear, and the bed I have will bolt on after I move the crossmember. Is that correct?