390fe knocks @ cold start up

That's the difference, piston slap won't go away until the engine gets warm enough for the pistons to expand and tighten up the clearance- that's why old-style forged pistons made from alloys that expand a lot sound like the pistons are trying to change bores when started cold. Loose mains will only hammer for a few seconds until oil pressure comes up, and the oil "wedge" is restored