My opinion:
For pulling the baler you are talking about, I would think you would be better off with a 2-wheel drive 8540 compared to a 4-wheel drive 7040. I would think the price not that different for those two configurations? For a tractor the size of a 7040, I would think 4WD would be a must with a front end loader. Absent the loader, I would not think the 4WD as important.
(Of course, I looked at the 8540 a few years ago and ended up buying a John Deere 5093E. I thought the JD offered more for the money then did the Kubota)