No matter what panel you buy you will need to trim it down, but my thinking here is once you remove the crossmember to fit the inner sill you may find more rust so then you would need the larger panel for length.
No matter what you will still need to trim the floor panel which ever one you buy to take into consideration the inner sill to first flute.
I understand where you are coming from ref the panel size etc but knowing how well our little cars like to rot having more material than needed can be a boon.
All of this is a recommendation however and the final choice is yours to make and I also understand there is the extra cost due to your location.
alright great! so can i take off the crossmember now to have a look? do i need to strengthen the car at all before removing? may as well see what's there and then order accordingly.
I don't see why you need to remove the crossmember to replace the inner sill, surely you can just leave it in place, remove the inner sill and carefully slice the floor away underneath it and then insert the new panel.
If the crossmember needs replacing anyway then fair enough but if its ok I would leave it as the less you remove the less you need to put back and the more datum points you have.
as sonik said above seems like both ways are possible but will see how it goes when i get round to install it. ordering my last few panels. hopefully can get stuck into it soon.