Used a router to round some of the edges. The end piece of the plenum will be welded to the main body with a cleaner butt weld, leaving a nice rounded edge along the rear corners.
With the plenum end piece hammer formed and some priliminary finishing done, I clamped the piece to the buck to perform some stress relieving to take the spring out of it.
Before the stress relieving, when I would remove the piece from the buck, it would try to spring back to it's original form of a flat sheet. After the stress relieving process, the piece more closely conforms to the shape of the buck when it's removed.
The stress relieving involved heating the piece with a torch to around 180-200 degrees F.
I'm not making any predictions. This sort of fab stuff is something that comes very slowly for me. I don't want to make any quick, wrong steps. It would be disastrous if I had to scrap a piece at this point.