I have bent my share of sheet metal ..
I made an entire truck body from sheet metal, see My Photos. Here's my advice. Make a full size model/mockup out of cardboard, or poster board, or illustration board. A hot glue gun or tape will work to hold it together.
Make two identical pieces of the sides and the front/back.
Obviously the lengths of the sides and the front/back will be the same. The bottom dimensions will be different of course. If you know those dimensions, start there. The height will determine the lengths of the sides. If you know the dimensions of the transition piece that will be another good thing.
You can also start with a full size drawing, but 2 dimensions is difficult to deal with and won't give you the angles in all views. You would need a side view and front view and possibly a top view for orthographic projection. I'd be lost using trig to make this, but I could make a easily make a mockup.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)