I believe you still have your drivetrain in.
That's good.
However, it probably wouldn't make any difference.
The drivetrain is loading the chassis.
When I did mine, I put 3 sets of jack stands in 3 rows to support the chassis evenly.
I felt it was the best way to support the chassis while replacing the quarter panel.
The dutchman is what the quarters ride off. I replaced it first.