Cut the
oil filter open---put in a magnetic drain plug
you say that you are still in the build process, meaning the engine hasn't been run much---that pushrod rocker may not of been seated in the cup correctly(with shaft rocker systems they are tricky to get all of them seated during the initial install) check others for marks around the adjuster
there is no reason to remove the heads but if you can't find pieces around the top of the head you might want to check at the corners where the
oil drains back and possibly remove the manifold
If it was during a race, I would put in a new adjuster screw and pushrod and go run it
Jerry