The gasket is sold as "fender welt", I bought mine at classic parts. What I did was after everything was painted I used 3m weather strip adhesive to secure the welt to the fenders, use enough adhesive so the welt stays where you want and then I bolted the fender to the bed. To make the curves you have to cut triangle pieces of material out so the fender welt lays flat. The welt comes in different colors.
http://www.classicparts.com/1941-72-.../#.UYhxtlet8uM