Well, I thought I would give some 'in the field' feedback, and advice. I bought my 1026r used this spring, and the 5' tiller came with it. I hooked the tiller up yesterday, and set out to create a new garden in our field. Our soil is heavy, and the target area was mowed field, 4". Fired the tiller up, and lowered it lightly into the ground. In low range I tried to go forward. The tractor rpms dropped fast and the tractor just about stalled. I then lifted the times, to 2" down, and started again. It was a slow process, with the tractor under some serious stress. I figured it would get better, as I made subsequent passes on the same tilled ground, but same loading on the tractor. The last passes were on dirt that had been tilled 2x previous, still too much for the 1026r, IMHO.
So, for those out there wondering if your 1025/1026r will run a 5' tiller, it technically will, but there is just not enough pto HP to run it properly IMO. I would rather a 4' tiller that puts less stress on the tractor. I'm going to sell the tiller and get a smaller one.