Just a different avenue to consider; I built a 428SCJ back when I was in college (many moons ago) and during the initial start-up, we had an issue with
oil leaking from the head gasket. If my memory hasn’t failed me, I think the problem was attributed to the improper application of the sealer to the steel head gasket. As you already describe, the
oil is coming from the junction of the intake and head so the head gasket may not be the issue but it is worth checking to completely rule it out as the problem.