That piece is called a "coolant elbow" and is very commonly broken when doing what youre doing....so common in fact that Dorman, maker of all those "Help!" products you see in parts stores makes replacements.
http://www.dormanproducts.com/p-19917-47065.aspx
You should be able to walk into most parts stores and grab one off the shelf, might jot down that part number.
The problem youre describing with the upper plenum is also very common, as are failing lower intake manifold gaskets, which Im assuming is why you have it torn apart to start with.
If youre replacing the LIM gaskets, youre time and money ahead to go ahead and replace the upper plenum and gaskets as well....vice versa if just replacing the upper plenum...ive been out of the 3800 world for a while now, but Dorman also makes an upper plenum which is supposed to be pretty good, be sure and check RockAuto for a price comparison if you dont mind waiting a few days.
GM came out with some redesigned gaskets several years back that were metal framed, I dont know if the aftermarket has since copied this or if its still a dealer only item....either way, new upper and lower gaskets and a upper plenum should probably outlive the rest of your cars' life.