EVERY THING Dave said. One thing I do is get the car up on stands pull the wheels and you can use a framing square or plumb bob the mark the floor for wheel centers. I draw diagrams with the measurements and go back to them while reassembly is being done. Tack it all together remeasure then weld it for good. I Always measure diagonely along with front to back and side to side. No one wants to drive a car that goes down the road sideways. Its not a hard job, but one that can go bad easily. Good luck and keep us posted. Pictures would be GOOD