Take measurements and mark reference points for the frame nut boxes. I used a framing square and made marks on the door sills before cutting anything out.
Cut the floor out in sections, tunnel and all, even a 3-4" margin from the rockers is fine. Once the bulk is out of the way it makes it much easier to get the rest out.
Little trick a shop told me was don't worry about the lip at the rear. Way to many spot welds to try and cut unless you're going for some sort of concourse resto. Just cut and grind it flush so the new lip lays on top, you'll never notice the extra layer in between.
Also, unless you're real budget strapped I wouldn't try and save the seat pans. PITA to cut out and save.
Get your new full pan before cutting out the old, that helps to see what you need to leave.
Cutting the rear frame loose from the pan was the hardest. Lot of spot welds that are tough to get loose. Spot weld cutter and an air chisel are the tools of choice, then side grinder and cut off wheel to clean it up.
A cheap hole punch from HF helps for prepping the new pan, not required but saves a bunch of drilling for spot welds.

Article in the Pontiac mag last month on floor pan replacement as well.