What they mean is they want you to "level" (shim) the car on the jack stands. What this involves is correcting for any possible unwanted garage/shop floor high/low spots where the jack stands are sitting (and/or any differences in jack stand heights) by shimming up any low jack stand to make up the difference.
Without shimming up any low stands (and/or low spots) before installing the cage, it's possible you will end up building in unwanted twist (side to side and/or diagonally) into the race car body/chassis when the cage is done.
You also have to decide where the "correct" spots on the body/chassis are to place the jack stands. If your putting a cage in a car that still has all the nose weight of the engine/drivetrain still in the car, putting the jack stands in the wrong spots could really fuuk up things & make the doors hard to open & close correctly after the cage is in.
Putting a cage in while the engine/drivetrain weight is out of the car makes jack stand placement a lot easier giving you more fudge room if your placement is a little off the mark.
460 street '66 Ranchero.......................finished someday.
460 race '70 Maverick..........................finished someday.
All 'glass Top Sportsman '69 Mustang......ummm, check back when I win big playing the Texas Lotto, or online poker.
page. Some ancient & newer local race video.
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