In my part of this fair country we call it the rust belt - and those screw on body snaps on a pickup were sure to create 40-50 rust holes in the top of the bed. The aluminum framed version is the only kind I'll use and have since they first came out. Any of my "before '01" trucks went either with a cap or nekkid (so I could shovel out the bed when we got snow).

This is what I have for my 06 F350 and this shows one for a '77 F100:
http://www.autoanything.com/tonneau-...&from_search=1

(They also got more expensive in the past year !!!! )