I would like to make a wood beam with pine board to cover a ductwork bulkhead in the upper corner of the kitchen.
If the goal is to make the duct look like a wood beam it can be done but it depends on how the duct was supported and sheetrock attached. If ribs were installed around the duct and the sheetrock attached to them it will be easy , if the sheetrock was glued to the duct it will take more work but still can be done. The reason I mentioned sheetrock glued to ducts is I have run into this several times and I would not hang anything on it until properly supported.
You can cover that soffit with w/e you want. But if you pull the soffit down, you'll have nothing to attach your fake beam to.
IMO it will look fake because it's wider than it is tall. You could trick me if it only covered the face and if it had a reveal on the bottom. E.g., that's a 10" high soffit; slap a 2x12 on the face of it. Now it's a beam ;)
A 4x12 would look even better, but it'd be too heavy and expensive. That you could fab up out of three pieces of plywood (or one-by stock if it's longer than 8'): 12" front, 4" bottom, and a 2" reveal on the back side.