One thing you will want to check is that your body was fabricated with the springs at the same location front to back. I have done many of these and the only cars that were close was 340 or HP. cars. Six cars and 318 cars were usually off. Did you ever see a car trying to do a burn out and the car keeps going to the same side. Take measurements front the front suspension to the rear to make sure your springs were tracking correct from the factory. K-member bolt to torsion cross member to spring mount to shackle mount. Get ready for a surprise.
You can use spacers to put the front perch in the correct spot. then you can use them to locate your weld in boxes. Also this would be a good time to move the rear back 1/2 inch for more tire clearance in the front of the quarter. I moved mine back a 1/2 inch and moved the lip front 3/4 and got 29's on the car. I can still put wheel well molding on if I want.
These are just tips you don't have to do them. Also make sure your rear shackles angle back away from the rear. From the top they should angle back to the bottom to allow the leafs to work. When I move my springs back 1/2 inch my rear shackles with ss springs ended up correct as per the Direct connection chassis manual with out using stepped shackles. I may have to make corrections when I install Cal- tracs split monos
Measure several times and weld once. This is not something you want to rush. Like I said did you ever watch a burn out contest and see cars that actually cannot keep the car from trying to spin around or even at the track. A lot of bodies are made out of square . Just look at the under side of some cars including yours and see where the leaf spring mount is welded on in reference to the hole in the bottom of frame from one side to the other side. also measure from the front spring eye hole to the rear shackle mount they should be Identical. Good chance they are not. Things like this is what has to be corrected on many production cars.
Bodies in white as they were called were near perfect when measured. as were all SS cars. Good luck , hope I wasnt to late for you to check things.
Also drill extra holes in the boxes 1 inch higer and 1 inch lower to corect spring angle if you have to, or control height. Do the same thing to your shackles Moroso sells the shackle kits. these holes front and rear make your rear leafs adjustable angle and height as needed. Without using a chassis manual these mods you are doing will not be able to be fine tuned.Just a Thought of concern I thought I would pass on. Steve