As Kevin says a Marsden nut is not what you want to use. It needs to be tightened to lock. You want a prevailing torque/deformed thread lock nut. So you can "adjust" the spring tension. Make sure you use the same grade nut as the Ubolt. Which is probably Gr 5. The American Classic Truck unit has incorrect hardware. They show a free running nut with lock washer.
The TF (55.2-59) Blue Chip GMC trucks use the same style spring loaded attachment. May be exactly the same. I have a set to look at.
A Marsden nut does "look like" a deformed thread nut.
Be sure your ubolt threads are nice and clean and that a regular test nut goes on OK.
Harold's idea is just fine, I myself don't like nylon locking nuts.
Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn
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