360 Magnum valve train noise

You've left out so much info that it's virtually impossible to pinpoint. It's GREAT that you have FI Tech though...... Anyway, cam specs, lifters used(new or used), pushrod spec, and rockers used is a great place to start. Otherwise it could just be a frickin muffler bearing.
I'm going out on a limb and guess whoever put it together either didn't get correct pushrod measurements, or, the lifters are dirty. As in, internally dirty. Either didn't get the used ones clean enough or didn't bother to open up the new ones to check for debris.