I occasionally see the d16 dual weber intake manifolds pop up on USA, Canada, and UK ebay. check for it. Its kinda rare as far as I know.
Else buy a d16y7 intake manifold, and hog out the throttlebody mounting surface to accept your carb of choice.
Could also find H or F series intake manifold lower piece, and cut right near the plenum bottom half, and weld it to teh d16 runners, use some aluminum welding to smooth transitions, and then build a dual carb adapter to bolt to the intake plenum half that is left open.
Spit balling ideas, because the carb pieces are rare, and might not be found!