Aircorpslibrary has quite a few A-20 manuals. From what I'm seeing over there, it looks like this fairing contains an extremely large filter and two ram air doors, and is just attached to the top wing surface. The main wing seem to continue normally while passing through the nacelle, just having cutouts for the leading edge, trailing edge and possibly the bottom surface. In front of the front sheer web is an alcohol tank for deicing. I can't send the images because they're behind a paywall, but look for:
-Erection and Maintenance for A-20G, A-20J, A-20H, and A-20K Series, AN 01-40A-2, 5-Oct-1944 pages 5, 21, 23, 28, 40, 41, 43, 89-91, 93, 110, 133-139, 144, 145, 214, 215, 224, 308-311
This manual also includes excellent station diagrams of the fuselage, wings, nacelles and stabilizers, diagrams of many other parts including many views of the landing gear, and diagrams on bomb loading.
-Illustrated Parts Breakdown for Douglas A-20H & A-20K, AN 01-40AP-4 pages 15, 24, 26, 28, 40-44, 50, 54-56.
You can probably find the manuals elsewhere as well. Hope this helps.