I'm always leery when it's up on all fours. I made special ends for my jack stands out of plywood so there is a flat surface against the car and full distribution across the end of the jack.
Some have used Hockey pucks for this purposes.
For extra safety I always use jacks at the other structure points on the end I'm working on. It's a little tight at the back
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2002 S - old school third pedal
Seal Grey
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