I work with metal alot, fabricating very random things. But enough about me.
You're going to want to build some substructure, small square-tube steel is my personal favorite. Running spars along the outer edges that fit inside the bed and cross-struts are the way to go. 'Member, x-patterns are the strongest, but I wouldn't suggest one long x from front to back, put a center strut in and then put x's in those smaller areas. Two beams evenly spaced from front to back if the bed is really long (I was born in the eighties, I have no clue what that vehicle is).
This also gives you a solid mounting area for the gas struts.
Also remember, anything you do needs pictures posed here for us to gawk at!:thumbup: