Good luck trying that one. I suggest you try a very short length. And the advice of others on other threads about cassettes might obtain: get a few cassettes from the local mini-lab that leaves a stub out of the cartridge, rather than opening the cartridge. Might simplify; and if you still have those scratches, suspect the film.
But as I said--having been there, done that--the Watson loader requires you to rotate the inner section to Open (I think that's the term--it's been more than 40 years since I used one) after you have attached the leader to the film cassette, closed the film cassette, and replaced the little door to the film cassette chamber.
Failing to do that means that you drag the film around the edges of the light trap in the Watson loader--it feels heavy going, too; which might be what you are feeling rather than crud and detritis on the film from its storage.