Massey ferguson had books called "product manuals" and I think these were for sales men back in the day and listed what went with etc
The little 135 Im pretty sure was capable of driving a 6 foot slasher if my memory is correct
Tractors worked a lot harder than they do today as most farmers have much more power in the same size tractors
Even so a 6 foot slasher would feel extremely heavy on a 135
You are right I believe. Back when I had the farm (oh how I miss that place) the Massey 135 handled this 6ft unit pretty well. I recall checking in the book before using it and it said 6ft was suitable. Going over contours (low ones only) it occasionally would pop a front wheel but nothing much. Was a great setup.