[elitecustombody]

measuring in X-pattern will tell you if the car is square,invest in tram gauge,you can buy one for under $200,nothing fancy,but it will do the job.You can replace the metal in whatever order you think is easier,I would cut it all out,fit the floors in with temporary clamps and screws,fit the rail,trace out the rail on the floor,prep the floor and drill holes in it,prep the mating surface on the rail where you'll have the spot welds,set everything back in,and start welding one spot weld at a time in different areas to eliminate any shrinking and always keep checking the alignment of all panels