I don't understand the thread title compared to your post????
Do you have a B7000 or do you have a B7100, that your trying to run the flail behind?
The B7000 PTO is under HP of what the flail mower requires for a min, in the 10 to 12 range.
The three point capacity is at 375 which is right at weight of that implement.
The B7100 is at 13 HP and a lift weight of 400lbs.
The normal width of implements for the b7100 was 42" to 48"
You will still be right at min HP yet under on lift with the 42".
Lift and HP are where rated when it was new, and I'll bet your 40 / 50 year old tractor is down a bit on both lift capacity and HP.