Sorry so long to get back to you, busy weekend with the kid's cheerleading competition.
Looking at what you have to repair on the lower section of the green cowl, you likely could form that area by clamping some sheet metal in a vise and tap over the folds using a nice flat body hammer. Then to add the shrinking for the "roll under", I would use some heat and quenching. The flat part in the middle we would want to leave alone, no shrinking. Then, by shrinking the sides in a wide vee, and the flanges in a rectangular to match the wide vee's lower dimension, we should be able to water cool and provide shrink. See link:
http://i5.photobucket.com/albums/y167/rmccartney/Metalworking/heatshrinking.jpg Although not visible, you would want to heat both sides and flanges at the same time for better rolling action. Trying to heat only one side will leave the other trying to keep everything straight. You don't need it bright cherry red, as this will tend to cause more scale to form, but try to get both sides heated directly across from each other before quenching, for best results. Leave some excess on the ends in order to trim later, trying to radius right at the end doesn't give you much to work with, especially if you need to resort to plan B, which is heating, stick one end in vise to clamp, LIGHTLY pull toward the shrink direction, then quench. If you pull too much, it will kink. But heat the two sides and flanges, quench, then check to the truck for fitment. Move as needed for the next shrink spot, whether that is 1/4" , 1/2", etc. The wide vee and rectangular spots will likely (and should) overlap before all is said and done in order to get enough shrink in there to form the radius you need.
Here's a picture of me using this process, albeit on a single side and it is shown a bit too hot as I wanted to make sure the vee pattern showed up for the camera...
http://i5.photobucket.com/albums/y167/rmccartney/Peter%20Romano%20Roof%20Repair/Picture100.jpg Here is the entire thread, just to show you where and why it was being used. This will be a time consuming process, but should get the job done with what you have at your disposal..
http://www.allmetalshaping.com/showthread.php?t=3634
