Next... I cut the entire rear sheetmetal out of the car, from the rear footwells to the rear bumper. Plasma cutter to the rescue, for sure.
I spent a long time making sure everything fit, and was square and plumb. I checked every measurement probably 5 times. Getting the back half frame rails in straight and true is critical to how the car will eventually drive and go down the track. I used digital levels, Starret machinist level, lasers, and anything I could think of to make sure it was in exactly the right spot before welding.