[STEELCITYFIREBIRD]

Piston slap is usually most pronounced on a cold engine and then quiets down as expansion equalizes .
With that I assume it possible, like Paul said; collapsed in service. But when you fit the pistons, they are either in spec or not, and this builder is noticeably obsessive compulsive for detail!! (good for you!)

A good test would of been to kill a cylinder at a time and see if the noise goes away or lessens. Wrist pin and piston slap noise are most prominent on the power stoke.

Condition that could be a contributing/casual factor for collapsed skirt in service;
Running the engine high rpm or hard under load or unloaded/free rev it high/hard... COLD. Commonly done to show your buds what your hot rod sounds like.

These long stroke motors usually have comparably short skirts, don't know if that amplifies this condition, but I theorize it would.