Short answer is yes, the bush hog will connect to the B21 PTO.
The real issue will be that the B21 is rated at 13.5 hp at the PTO, so you will be very underpowered. You can run a 6 foot bush hog, but it will be slow going.
My B21 spent the last few days running a John Deere 506 bush hog. It worked fine in tall grass. Slower than one of our 40 hp farm tractors but easier to trailer to the church than they would have been.
My only dislike is that lower link mount points are so high on the 506 that the B21 could not lift it very high. If I were using it with the B21 again I think I would install a second set of lower link pins lower on the 506.
If the JD 616 is a 6' mower it is too big for the B21. It is a great tractor but rather weak in the PTO horsepower department. The rule of thumb is 5 PTO hp per foot of mower. I use a 4' mower on mine and it can bog down in thick damp field grass.