It really screams to get the monoplane treatment in the same way Hawker used the Fury as a starting point for the Hurricane.
Perhaps marry up the fuselage of the B.534 series IV to the wings of the B.135.
The landing gears of the B.135 were far enough apart that you could put something on the centreline,
Also, if you keep the four machine guns in the fuselage as the B.534 had them, then the B.135 wing should have enough room for a higher calibre gun on each side.