I disconnected the shell from the channel, but I was dealing with a smaller trailer. Having the shell off can make further disassembly easier. I wouldn't recommend leaving the floor attached to the shell for lifting. I'm assuming the belly pan is still in place?
Before you lift the shell, make a pattern of the floor inside the channel. The footprint isn't exactly symmetrical, so matching the original can make the reconnect easier. If your floor is really in good shape it may be sufficient as patterns for a new floor.
My patterns for the straight middle sections where 48" front to back so they would match the width of plywood for the new floor. I moved crossmembers around as I installed 4 tanks under the floor.
Cross brace the shell to maintain the original shape (or close to it). A shell that long could buckle if not supported. Some remove the door also (braced again) as its heavier and may make the shell tilt during liftoff.
Tie the shell down to the ground somehow to keep it from movement in the wind. My gantries were metal pallet racks 12'X12'X3' so I strapped to them.
I have a car rotisserie so I didn't need the gantries for the frame.