Err, the P-3 currently has both a weapons bay and sonobuoy launchers. The weapons bay is just forward of the wing and aft of the APU installation (if you see a picture of an EP-3E, the "M&M" mounts in the weapons bay) while the sonobuoy launchers are in the aft fuselage. Working on the P-3C--> EP-3E conversion gave me a nice familiarity with the basic aircraft. I could see deepening the fuselage giving you a more spacious weapons bay as well as more fuel tankage, but, really, to make it useful, the upper lobe needs to be widened, it's rather tight, especially on an EP-3E that's crowded with ELINT crew and electronics.