The mule drive should be able to move a little bit within the limits allowed by the spring that runs to the S-piece. That spring is meant to provide tension to the long drive belt that runs from the mule drive back to the mower gear box. The mule drive should not move to the back of the tractor - if it did, the drive belt would slip. Instead, it should be pulled forward by the spring to the point where the mule drive is just about 1/4 inch short of hitting the frame (you will probably see a dent where it has rubbed in the past). If you used a different hole in the support arms to mount the mower, or if you turned them upside down, it is possible that you mounted the mower a bit further back than the time before and this would result in the spring being a bit tighter.
The position of the mule drive doesn't really affect the tension on the shorter belt running from the engine pulley that much. It is the idler pulley that makes the big difference. From your picture in post 12 it seems that the idler may be set a little low, and the finger meant to stop the belt from slipping round when it is disengaged is also not in its proper position, but to me it looks mainly as though the belt is at least one size too big. Looking my own, the fit seems to be tighter than yours - even when disengaged. I think you need a smaller belt.