Full Floor R&R 70 B Body

I need to replace the full front and rear floor pans plus trunk pan on a 70 RR, is there a need to level and support the body on jack stands to do this? Or can it be done without a concern for sag or twist in the body? And which needs to be done first, looks like the rear floor pan sits underneath the rear flange of the front pan and the trunk floor?

My first time to try this but am sure many of you have first hand experience at this and know the pitfalls.

Jim