They repeatedly air dropped the Shuttle Enterprise from a modified 747 for approach and landing tests. A roughly 5x greater load than this bomb. Design a single use airframe around your penetrator bomb, perhaps with a propulsion package, that can be separated at an appropriate time. Might even provide a limited standoff capability. More complicated but more capable. You could have an even larger bomb, or trade size for standoff range. One problem would be how to secretly do adequate testing. I’d hate to depend on getting it right the first time. The other would be how to get your loaded 747 close enough. I doubt it would be very stealthy.
I remember that, on the back of the 747.
Design a single use airframe around your penetrator bomb, perhaps with a propulsion package, that can be separated at an appropriate time.
Why not just hang the unit from the bottom of the 747 instead, like a belly tank? It's a long and slender package and the weight would be below the centerline instead of above it.
I don't think it needs any propulsion, just a guidance system. Gravity does the rest.