I would take out as little as possible. I am currently putting a new floor in and i had a very small rectangle patch for an overlap in the firewall to floor area that needed to be replaced 2" by 6" (where the seam from floor to firewall). That small patch is a lot of work if you plan on hiding all the weld lines (grinding smooth). Depends what kind of restoration level you are doing.

Structurally you need to brace the car very well especially if you start repair/replacing "A" & "B" pillar sections There will be nothing holding the rest in place.