Looked at a 2004 Grand Prix on the showroom floor, with the cover still in place, from the PCV valve end it looks like the series II manifold except aluminum. The dealer had a 2004 Regal next to the Grand Prix and it made it easy to compare the two motors. The injectors and the throttle body look different but that is all I could see with the cover still covering the manifold.
If GM did not change the bolt spacing on the upper manifold there may be a chance to adapt it to a 3800 II (L36).
I f anyone has a 3800 III motor or has a friend with one please take a picture and post it.