It took me a year, but I finally got this attachment figured out. Cub Cadet made an adapter package to convert the IH tillers for use on tractors with a GT Category 0 3-point hitch. It seems very few were made, and I had to work from a picture of an existing one. All of the parts are NLA. I sold my 128 and tiller after I picked up another 1A tiller with both extensions, with the hopes I could put it on my diesel and have one less machine to maintain.
The first step in the process was to make the part that bolts to the tractor itself. I'll note that the tiller technically doesn't use the hitch because it isn't using the arms. It latches onto the hitch plate and there are lift links that connect the hitch rockshaft to the tiller lower arms. The gearbox mount looks like this:
This latches on the horizontal rod on the piece in the previous picture, and then a clevis pin goes through the top part of this and the top link bracket, firmly holding the gearbox bracket and tiller in place.
Below is what it looks like all attached to the tractor, with the lift links in place. The part on the tiller the lift connects to bolts on where the lift chain used to be. The gearbox bracket had to be notched for the hitch lift bar, and also for the belt. More on that later.
The long belt is also NLA, and I decided to add a jackshaft underneath the tractor for several reasons. One, I wanted to be able to use my 50C mule drive with its offset pulleys. I also wanted to avoid buying a really expensive long belt that I'd use maybe twice a year. This setup allows me to keep the deck mule drive and belt, and requires only the addition of a ~$22 belt. I used a belt tensioner removed from a newer deck I parted out. This bolts to the tractor using existing holes.
Notice the offset to the left side of the tractor; 2.5" of offset was necessary to get the mule drive to line up. Not visible here, but I had to extend the left side of the left notch in the tiller gearbox bracket about 1/4 to clear the belt that now comes in slightly from the left instead of straight.
All in all, it works great!
The diesel is a nice tiller tractor- it has plenty of power, and the hydraulic lift is great for raising the tiller. My only complaint is the tiller pushes the tractor a bit. That's inherent to any hydro tractor, and it's just something to live with. It is really easy to install/remove the tiller since the gearbox and tiller stay assembled. I just have to back up to it and set it on there.