It is certainly possible for valves to tighten up on their own as well, from one of two mechanisms:

The pushrods in a 235 rotate because the cam lobes are off center and it makes the lifters rotate. This is a clockwise rotation which is the same direction the adjusters rotate to tighten. If there is enough friction between the pushrod and the adjuster, and you didn't tighten the locknut, it is possible that the pushrod rotated the adjuster.

The other mechanism is that the valves got too hot and the seats receded into the head from overheating. This is a runaway condition because it tightens the clearance.

Either way, readjusting the valves fixes it unless the seats got ruined and leaky from overheating.

If the truck sat for a while the carb could start leaking independant from the valve problem, but a tuneup always starts with a valve adjustment, since a problem there will mask any carb or ignition problem.

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