The civil PBY Super-Cat conversions switched out the P & W R-1830s for the Wright R-2600s from a B-25.
When Terry Moore built the Super-Cat from the film
Always he used Monogram B-25 engines/cowlings
on the Monogram PBY. Due to the larger diameter of the R-2600 he did have to build up the nacelle aft of
the cowlings.
So as the Monogram C-47A/DC-3A kit has R-1830s, (note that DC-3s and C-47s had the Wright R-1820) do
the same thing using Monogram B-25H/J engines/cowlings. Then you can use the R-2800s/cowlings from
a Monogram A-26 to convert your engine-less B-25H to the one-off NA-98X.
BTW the Monogram A-1 R-3350 cowling has long been pointed out to be undersized, so it could work as
a new design R-2800 installation.