The 747, like all fixed-wing powered aircraft from the Wright Brothers' Wright Flyer I depends on lift from wings flying through atmosphere to fly. This means generating lift as air moves over the wings. At some altitude, the thinner air means insufficient lift. So wings don't help you above that maximum. You'll notice that there are no large wings on any space craft that is now or has ever been in service. There's a reason for that.
Yes, the Space Shuttle had wings. But they were stubby and didn't provide much lift compared to the 747's.
Your engine must provide thrust. As you climb, you lose thrust. Especially from propeller or jet engines. So you have to compensate with a different form of thrust. Today, that means rocket engines instead of jet engines. Your 747's jet engines are useless above some maximum altitude, because there's simply not enough air.
(See this post which covers thrust and lift in greater detail.)
Rocket engines consume a great deal of fuel. Basically every machine humanity has sent to space has carried more fuel than payload at the time of launch. Wikipedia has details to expand on this.
Space ships steer by either rotating to point their main thrusters in a new direction or by using smaller, engines to point and fire, shifting the direction of flight. Your 747 flies by moving control surfaces to change direction. Those control surfaces have absolutely no use once you rise above some altitude. For the same basic reason that you lose lift.
Airplanes maintain cabin pressure by bleeding air off from the engines and circulating it into the cabin. This, by definition, means the cabin is not airtight and that it must, by definition, have access to external air. Your space-747 must have oxygen on board and a sealed environment to maintain internal air pressure against the vacuum of space. That's a completely new set of systems that can't just be welded onto the fuselage; it's a complete redesign of the whole system.
Space craft must be shielded against radiation that simply isn't a threat within the earth's atmosphere. Your 747 would need some serious upgrades to handle this.
Literally every exterior component of your 747 needs to be replaced, from the glass in your windows to the aluminum of the hull. Nothing is of sufficient strength to handle the new stresses you're wanting to introduce.
Your plane can regulate temperature because it is flying in an atmosphere. It's a relatively trivial task to heat or cool the outside air as needed before feeding it into the cabin.
But the vacuum of space is literally the best insulator possible. Wikipedia describes the array of thermal control systems required to maintain temperature in space. A 747 needs none of this complexity. Each of these systems would need to be factored into your 747-like ship.
The 747 was introduced in the 1970s. Even with upgrades, the onboard electronics in it are not going to cover the needs of a space ship. You will need to pack in far more complex power generation, communications, navigation, life support, avionics, and other computer sub-systems. Other than whatever onboard entertainment, the phone that lets flight attendants talk to the cabin or to the pilots, and overhead lighting, I suspect there's literally nothing in the onboard electronics that would be usable in the new space role.
A simple across-the-lap safety belt, barf bags, and flotation devices are not going to cut it. You need pressure suits. Which means you need seats with enough room for an average human in a pressure suit. There's not a 747 in commercial service that has the room you need. And I seriously doubt that flotation seat cushion is any use at all.
Basically, there's no advantage to a 747-style plane for space flight. You'd have to strap on rocket engines, massive fuel tanks, and all the attitude jets required for steering. Your wings can never create sufficient lift at low altitude to get you above the atmosphere. They would just create drag once your massive rocket engines begin to accelerate. They probably couldn't be made strong enough to overcome the forces experienced during reentry, either.
Basically, the only way to make your 747 into a space plane is to add handwavium in massive doses. You need some sort of anti-gravity engine to negate the weight of your plane, some sort of engines that require little or no consumable fuel to provide lift, and you need steering engines added, too. Oh, and radiation shielding and other environmental concerns. And even then, the final product probably wouldn't look anything like an actual 747 at all.