Darvan, digressing slightly, the lay down attack would be nearer 415kts, even in training we did 375.
Time of bomb fall was in excess of 5 seconds. Retard delay was 32 seconds. Distance from GZ at detonation should be in region of 4 miles with the aircraft opening range. I estimated the aircraft would be about 5.5 miles from GZ when the blast wave overtook the aircraft with an pressure of less than the critical 1.25 psi.
The variant carried by the Bucc had a much lower yield.