Rich, I wish you luck.
I have been doing some research as I too and faced with the same dilemma. I want to go with
THEFUELBOX FTC60T 57 Gal fuel tank tool box combo which will be .5 inch lower than bed rails, and 3 inches narrower than the bed rails, and the lid will be a total 8 inches narrower than the bed rails. This should give me enough room for a cover of some kind if it doesn't hang below the bed rails above the lid of the combo box. Now if I can get that figured out, the combo box will need the first 23 inches of the truck bed (it has to attach to the bed floor and front part of the truck bet with only 1 or 2 inches of space. So that means ,I will need the first lid of the bed cover to open backwards at least 27 inches so I can reach in and open the latches to the combo box.... MAN, I've Have a Head Ache Now !!!!