I'm no expert, but the collective has told me that a dual transmission is required for the roto tiller. Appears you need the very low gear only possible with the dual. Is your tiller like the one I have in my Avitar? If so, that is powered by the front pto.
Fortunately you may be able to modify yours to a dual transmission. Not cheap, but I have seen them on e-bay on parted out units.
When I use my tiller, I always use it with both transmissions in first gear which makes it just creep along. This requires the second transmission. You may still be able to use yours in first gear and ride the clutch every few feet.
My tiller is the front pto type it is on a 1979 2418. I might have a source to get everything to convert to dual trans. I have a friend with about 25 diffent tractors for parts. I think that I will hit him up about the trans parts and see if he has a K361 that I can rebuild and put in mine. This would give me a short down time.