I always use the rockers front to rear.Side to side I use a piece of square tube across the front and rear frame rails. If the level points are slightly different, split the difference. Put the car on jack stands on the frame and use cheap vinyl tile under them to level it. I built some adjustable stands for under the far rear frame rails to keep the car from sagging. Works great for me. Rhodesracecars.com make nice,reasonable cage kits. They will do any modifications to the cage you want.