OK, I haven't looked through this entire thread so I don't know if this has been mooted or not but...
For a naval P-38 (P-38N/F38F-1?), why not keep the basic airframe layout but move the cockpit forward in the centre nacelle & the guns back, set in farings similar to the B-25 & B-26? Not sure how many guns could be fitted but I'm guessing either 4 or 6, including 2 x 20mm.
It would give excellent visibility over the nose, considerably better side vision, possibly a slightly woozy flight for the pilot (being further from the CG) & not look quite as pretty.
:)
Guy