I'm gonna stop you right there.
The only two things which generally cause "piston slap" are:
- improperly sized pistons to the bore
- forged pistons (which expand from warming up at a slow rate relative to the other components)
The F150 never had 4.9L engines which had either, in the 31 year run (1965 to 1996).
Whatever noise you're hearing, I seriously doubt it's "piston slap".
NOTE: The late 60s / early 70s did have some HD versions of the 300 six which did have forged internals, but those were only installed in very heavy service dump trucks, etc. Never into an F150.