Why not gather up the specifications, and have a quality machine shop make you a power plenum to fit seamlessly with your Streamline? It would seem that if you take the DL power plenum with you, along with the action, and thread pitch/ size etc. it would be a pretty straight forward piece of turning. OR if the Threads of the DL power plenum are the same, couldn't the excess threads be parted off to fit the SL action block? You would lose a bit of internal volume. Another option is to have an aluminum spacer made to take up the excess space, that may be the easiest rout.
I think if it were my project, I would see about having a custom power plenum made. It may or may not be worth it depending on the hourly rate of the local machine shops in yer area.
Just a thought,
-Brandon