The bottom of that cowl appears to be what we refer to as a "hat" section. Looking at a cross section view, you have a flange, then go up, across, down, then the other flange. Looks somewhat like a flat hat...
The problem you will face in doing this in one piece is that the sides will need to be shrunk to add the radius to match the panel on the truck, and the flanges will also need to be shrunk adjacent to those same shrinks. Or make it in more manageable pieces and weld it together. Or pony up the 60 and use what you need from the store bought version. This will boil down to one of those what is your time worth things.. If you had to buy shrinker-stretchers, you'd spend much more than the 60, if you have someone nearby that you could borrow them from, you're ahead of the game, or you could make your own tooling, as shown here:
Here is a thread that describes making a windshield opening corner, much similar to the one in your truck. The tucks shown help to keep the bulk of the panel flat, and the "flange" for each piece has a radius that is welded/joined in the middle. What you are attempting is going to use shrinking of the sides to introduce a radius, and keep the sides (of the hat) flat. So it's similar but a different orientation, if you will.
http://www.garagejournal.com/forum/showpost.php?p=3511705&postcount=401 If you want to sacrifice a pair of vise grips, this will show you how to make tuck shrinking pliers that will be more consistent...
http://www.garagejournal.com/forum/showpost.php?p=1024584&postcount=101 
