Here's an idea for ya'. The '88-98 Silverados had a bed already shaped as you describe called a Sportside. The center box portion is steel, the outter side panels are fiberglass. And you won't have to fight getting the tail gate narrowed, which would probably be the toughest part of the job. If you search the yards you can probably find one fairly reasonable, and they fit the earlier chassis. This is the best picture I could find, it's a little chopped off the back, but you should get the idea.