Early A floor pan
As you can see, this is far more than just a matter of patching a hole. Here's the pair of floor pans I ordered, which several people kindly suggested. At first my plan was to use these as the basis and fabricate around them, but it's such a large area that I'm thinking it will be better to fabricate - or replace if possible - as much of the pan as I can. Sure does make it easy to get at the transmission linkage though!