I have the car on a spit and full bracing is attached. I'll have the door step / a post in place before proceeding to the floor

Has anyone tackled this job before and if so have you got any tips or pitfalls to avoid?
I was going to make sure the rear sub is in place before removing the old floor, and then use this as a datum for locating the new floor.
As I have the car on a spit, would you turn it upside down to allow gravity to support the weight of the floor when welding?
Do I need any additional bracing before removing the old floor?
Would you drill out the old welds from the inside or underneath the car? I.e. When fitting the new floor,Which panel should have holes in it for plug welding?
Thanks Chris