Its a good discussion and important to try and figure this stuff out.
In theory, that makes sense. Question is do you ever hear piston slap? Ive not heard any 350 owners reporting that problem.
Also, the amount of swelling the piston would have to experience to make this logical would be so much, you could actually see the difference in piston size cold vs. warm. That in and of itelf would present a ton of other issues. Even a motor with cold start piston slap is going to have a piston damn near perfectly perpendicular to the combustion chamber once it reaches TDC.
To test the theory, we can look to an engine design with known piston slap issues are the early Chevy LS motors. Wife's 6.0 suburban slaps every morning for a minute or two as it warms up. This motor has 192k miles and uses about 1/2 quart of 5w30 between 5000 mile oil changes.
One of the guys who's motor blew and he was tearing it down showed evidence of piston slap (youtube vid) and I've heard a couple of other guys in forums claim they have it. I have not yet heard.
I don't think we're talking about huge dimension here. If the piston isn't perfectly centered and level in the bore, even a miniscule amount would change the ring contact with the wall (that's the bitch of a circular contact, tilt it even slightly in any direction and you now don't have perfect contact anymore or at least not the contact the ring seated/wore to when it's operating normally.