No you should not have to cut down the PTO shaft. I bought the Herd Seeder when I had my TC21D, had to cut the shaft. Traded my 21 in for a 29 and had to buy a new shaft. So the one that comes from the factory should work just fine for you.
A hint, take your slow moving sign off when spreading for ease of grabbing the on/off arm. It gets in the way some.