Fairly normal, if everything was machined,assembled and clearened properly and it goes away once you're up to operating temps it's 'normal' for these type forged pistons.

OEM piston typically have a pin offset that reduces 'slap' during warm up. If it makes you feel any better 426 HEMI's used the rattle pretty good (like a coffee can full of Rocks) from the factory, "Typical cold-natured Chrysler Product' the old southern mechanics used to say....

You responded to say you're still hearing it after warm up, you may want an experienced stroker/hp motor guy look at that and advise you, could also be a timing/fuel delivery tuning issue but can't (obviously) diagnose it here.