To start bracing this project you will need six points for the car to rest on so the whole car will not move up and down at all at any point along its length. So you will need six axle stands, two at the rear on the outermost rear chassis rails then two more close to underneath the front pillar posts and the last two should be the screw type axle stands that are placed near the front bumper bracket areas. This way there is no flex or sag in the horizontal plane during work. I am assuming you will save the front pillar posts, if so, then you will need an "X" of two lengths of bar in a cross pattern between the two front pillar posts to stop any side flex. Then you need a brace in each front doorway at near the height of the front pillar posts going directly back to the middle pillar post of the car. That should be plenty to keep the dimensions of the car correct, but measure everything three times over and keep a notebook of all the measurements and check them each time you cut something out and also weld each piece in.
That should be a good starting point for you.
