I just reread the op, you say you have a bad cat? Define bad - is it blocked or collapsed? Restriction in the exhaust like that could help to explain the 60mph limit flat in 5th. May have nothing to do with the clutch. If you're replacing the cat anyway because it's bad this may not hurt emmissions so what I am about to say will not be too bad. Drop the cat and poke something through it from inlet to outlet a few times. That will "open it up". If you run better and can hold higher speeds better afterwards you have your answer, after all it's slated for replacement anyway.
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