When my 428 Cobra Jet was cold and the temps were colder, I would run about 80-90 psi with VR1 and a mechanical gauge. 100 seems a little high. Early on I heard a sound that sounded like the valve lash was too loose. Sounded like lifters. Not quite sure of the squeal, could be the fan belt is loose. I would suggest removing the valve covers and check your lash. Are you running iron heads or aluminum? The valve lash will be different for them cold, as aluminum expands more than iron when hot. After the start-up the valve train should be covered in
oil. If not, you may have something blockage an
oil passage.
I would try a different
oil pressure gauge to rule out a faulty gauge.