Unless you know body is "dimensionaly correct" now or will have it mounted on a frame rack all the while, you need to go about it in a rational way. i.e. Do cowl w/o disturbing any other panels. Replace a rail or a section before trunk/floor pans are disturbed. Etc.

Older unibodies have a "build-in stress" to them, best I can describe. Remove a quarter & watch as the tail panel moves. Cut out an inner & rail can shift.

Get some trammel points & long shafts to measure key dimensions; along/across/diag.

If the cowl panel(s) can be repaired in place rather then removing, you'll thank yourself in long run. It's the worst area of car to work on.

In summary, guess what I'm getting at is don't find yourself with a body that looks like it been picked/butchered by metal vultures.

Attached pic is typical example.