What is your ignition advance? What is your compression ratio? Are the pistons loose in the bore? The oil pressure is good which leads me away from excessive bearing clearance. The fact that it gets better as the engine warms up makes me think it's piston related. Are you using forged pistons? Forged pistons expand more and must be fit loose in the bore so they have room to expand as they warm up. When the engine is cold, the skirt will slap the cylinder wall making a diesel noise. Did you use hypereutectic pistons? I have heard stories that some brands of pistons run tighter clearances than standard pistons, but some shops forget to take this into account and machine the bore for standard pistons. This results in piston slap/diesel sound.