The H always had the abbreviated gun nose with 75mm gun and four .50 machine guns, though field conversions sometimes stripped out the big gun for more .50. The J could either be equipped with the glazed bombardier nose or a gun nose with 8 .50 machine guns; in both cases, the nose was longer than the H. The layout of the rest of the plane was similar or identical for the two types (defensive gun positions, packaged guns beside the cockpit, etc.)
The Marine Mitchells could be PBJ-1D, -1H, or -1J, and they all differed in detail from the USAAF equivalents, particularly in location of radar. If you want one of those Navy blue on blue schemes, the Marine Mitchell is what you want.
Mark Schynert