How many stations/attachment points on your rotor? I noted that Ford, like Caroni, sold rotors that had many stations for fine cut knives and another rotor with fewer stations for rough cut. At one point you mentioned heavy duty knives so I assumed rough cut rotor. If you have the fine cut rotor it's possible you cannot switch to the clevis mounted blades.
By the way, for all concerned, here is the FM220398 knife in question.
SINGLE PIECE BLADE
Most of us would refer to these as scoop knives. They are generally used as finish mower knives, not rough cut. Finish mowers aren't really expected to mow knee high grass so that may be a part of the problem.