This is basically exactly where I am at. Unless you are positive the original shell is correct I wouldnt bother with the bracing. Your not retaining enough of the shell for it to matter especially if your replacing the bulkhead as well. I tried fit it all together using bracing points from my old shell and its just caused more issues for me.
Not mine, but Im thinking Im needing to strip the cab back to this stage with some extra bracing and itll be minus the front bulkhead too. Then, with the new floor and load tray in place, lower the cab section on to it and go from there adding a new bulkhead etc. What I will do though is leave the current door steps on to give the cab an extra surface point on the new floor.
My advice would be to lay your new floor and load bed section out and align the two using the rear subframe as a jig. Make sure the two sections are square and temp fix them together through the top of the heel board with panel pins or tek screws. Bolt the new bulkhead to the front subframe with solid mounts so theres no play and offer it up to the front of the floor section make sure its square and centered and again temp fix it together. Make sure the whole floor assembly youve just created is level fore and aft and side to side and level up the front subframe as well then try the remains of your cab section on top and see how it looks.
Much appreciated carthorse, and thats a good shout. Ill be putting some extra bracing in so it doesnt completely fall apart in the meantime. Will also help it sit better on the pallet I have for the moment, but will follow your advice re the build element.
Subframe almost stripped down so will get that all cleaned up ready to use. Will be nice to be repairing something rather than taking apart! That said, Ive been working on repairing the passenger side door which bar some surface rust treatment and paint, is ready for the skin to go on. Will do the other side too so that when it comes to aligning door posts and pillars, Im aligning to fit the doors that will go on.