When I talked to a body guy about the same thing, he said to take multiple measurements and then go about it, never really said to brace it, just make sure every measurement is the exact same when the new panel is in.
If you do this ,,, you can't go wrong. I would (and DO) release some pressure by jacking up the car a LITTLE from the pinch weld area on the rocker panel.
Then Install the new panels,
Remember something,,, E body fenders will move approximately 1/2 inch in all
directions upon installation. (this is a GOOD thing) . you shouldn't have any problems.