I would assume the designers had a reason for picking the 747, maybe because it’s faster, bigger (about 50 feet longer) and can fly almost 5000 nm further.
Not sure what you mean about ‘overaged’.
Old design or just old?
The 747 is still in production, although they’re talking about winding it down. So it would be a new plane.
As far as old design, I would imagine that they would plan on starting with a stripped airframe anyway.
Whoever did that drawing for PM wasn’t an aeronautical engineer. Good luck on firing a load of missiles from behind the center of gravity.
If you can successfully get a vertical launch without damaging that huge tail, you’ve just changed that 747’s pitch trim to “crazy nose heavy”.
Not great.