NASA has 2 Boeing 747 as a shuttle carrier.
The first one was an American Airlines B747-123 and the second was a Jal 747 SR-46.
They were stripped of all airline equipment and the fuselage was reinforced to support weight of the 68 ton shuttle. The additions to the roof and the tail reduced the aircraft performance and its limit now (with or without the shuttle) to maximum Mach 0.6 (around 250 knots) and 26,000 feet.
If I remember correctly it can’t fly coast to coast non stop!
The gross weight of the 747 and the shuttle is 584,000 lbs, 264,900kg, and the maximum take off weight is 710,000lbs, 322,700kg.
The shuttle is mounted on the back of the B747 with a positive angle of attack to provide some lift.
Long live the B747