You are doing a "shell off" with the shell in place, which, to me, is the hard way of doing it. Build the gantries, lift the shell off, pull the trailer out, and then take it and get it repaired. When you get it fully repaired, you can use your gantries to lift it flip it, and you can then remove all the rust, paint with POR-15, install the new floor, then flip it and install the insulation under the floor, install tanks, install the new bellypan, and the new axle, and then finally flip it back over and roll your completed chassis underneath the shell. The beauty of this is that you spend minimal time on your back with rusty scale or drill cuttings falling in your face.
Have a look at the following thread:
Post 9 and 11 show some good pics of what can be done with the gantry frames. There is also a diagram for building the frames in a pdf at the end.