Here's an example of the frame work mounted type I mentioned that is made for your '77 (they also have the short bed version if that's what you've got). The good thing about these is you don't have to drill holes in the body that can lead to rust. http://cgi.ebay.com/ebaymotors/SNAP-...spagenameZWDVW

If you must have the type that snaps to the body then you're most likely looking at custom made as I haven't seen then commercially in about 20 years.