Glad to know we agree on this paradigm, bro.
Technically, you cannot modify a Cargo Plane into passenger transport. Well, let me rephrase this, you can modify any cargo plane for Passenger Airliner, but it will end up better off designing a new planes just for this purpose.
Cargo Flight and Passenger Flight have two distinct different. The center of gravity of the plane and the structural dynamic of the plane.
Cargo Plane have an empty hull, basically the center of gravity is on the dead center of the plane, and while passenger flight would normally tilted forward or backward because of the Passenger Compartment and Cargo Compartment as well as Fuel Storage and avionic.
Cargo Flight also have flight characteristic design to take heavy loading, meaning harder suspension and stronger tyre. And the braking can also be hard as you can shake the cargo violently but you cannot shake your passenger violently.
Basically, without talking about a whole lot of Avionic, Flight and Aerodynamic. If you want to use a Cargo Plane and use it as a passenger jet liner, unless you don't change anything and basically without anything in it, you will then need to move the center of gravity (mean redesigning the flight line) to offset the weight you added (Toilet, Cabin, insulation, Pressurization equipment, Fire Proof Bulkhead and stuff like that) And then y ou have to replace suspension, engine, brake, ailerons, flap, stabiliser, actuator and tyres for flight comfort and new aerodynamic result from the change of gravity.
Or you have your president ride on an unmodified Y-20, that's like have the President of the United States transported by a C-17 everywhere....lol
