Metal in the filter....tiny metal bits, or is it like a paste or peach fuzz? A little bit of metal is normal, especially on a new engine breaking in, but it should be like peach fuzz or a paste (often collects on the magnet on the drain plug). But it should be a small amount and there should not be any particles that are large enough to see as being a piece of metal.
Did you break the cam in with break in springs, or the springs for operation? Flat tappet cams these days have all sorts of problems because most of the oil these days is ****ty. And with the more aggressive cam profiles these days even when everything is right, flat tappets can still have issues.
Who cares how much horsepower it has, all that matters is how fast it goes!
Untested 331, lots of suspension, chewing up corners.