I would cut just as much as you need to to have solid metal. that way there is less fab work and the car stays more original.
as for the patch itself Don't know of any, maybe a chevy piece would work or atleast be close.
you could always make the piece I had to make my toe pannel because they don't make it. you can see pictures of it here:
you will see the roughed in piece in the picture, I need to update the thread with the finished piece all welded in, looks much better now.