While relatively inefficient compared to an implosion device like Fat Man, not to mention very heavy (because of the big "gun" barrel) compared to the yield, it was also much less likely to malfunction when used to destroy an underground target like a submarine pen. The Mk 8 could reportedly penetrate 22 feet of reinforced concrete before detonating.
The center pylon and belly of the AD-4 had to be modified for the Mk 8 because its weight, about 3,250 lbs, far exceeded its 2,000 lb design capacity, and its suspension lugs were 30 inches apart compared to the 14 inch distance between the latches on the existing Skyraider center-line bomb rack. Since the two forward spars of the wing weren't far enough apart for the longer rack required, instead of being embedded in the bottom of the fuselage it was located below the AD-4B's belly and housed in a streamlined fairing. In addition to beefing up the structure to which the bomb rack was attached, the modification also included the creation of a recess in the belly to provide clearance for the Mk 8 tail fin.
The recess was closed off by a panel when a Mk 8 was not being carried.
The external center-line pylon was retained for the AD-5 and -6/7.
Thanks to Ed Barthelmes for his help with documentation needed for this post.