1. 747s are civilian aircraft without the redundancy and strength requirements of military aircraft. Loading one up with nukes and waiting for one to inevitably crash is incredibly stupid. How many 747s have already crashed into mountains/piled into the sea so far?
2.When one 747 is used for carrying nuclear weapons ALL 747s become potential targets by enemy forces. How would you like to be aboard a civilian airliner when the "balloon goes up"?
The range of an ICBM should negate any disadvantage the 747 has; How many countries can target/track a 747 launching missiles 5,000 miles away?
According to Wikipedia, there have been 60 hull-losses of 747’s, less than 4% of the total built. And many of these were hijackings, blown up, stuck by lightning, hitting another plane on the ground (Tenerife), etc.
Possibly a better record than the B-52’s.