First I think it is a lovely image.
As for how it was done, I think I would have used a 120 mm lens for more coverage on the 4x5 and moved back so to frame the image as it is here, taking note of the groupings of flowers. You would have gotten a more realistic image without the fall off, plus the feel of the flowers being as close with the longer lens. I also might have waited for a lower angel of sun light. Still it is a nice image.
I personally have a 65mm super angulon I use for just such a purpose. I intentionally move in close so to get that distorted effect and use it creatively. The lens just barely covers the 4x5 at neutral. But I do not get any blurring at the edges or corners even stoped down to f/45.