I can't believe nobody has mentioned the HV pump. You are pumping all that oil right up to the top of the engine and it probably has had no drainback work done in the heads or the lifter valley. The fact that it's 20w50 only makes it worse-just returns to the pan that much slower. Under 3500 rpms may be the sweet spot where it just returns fast enough to be alright. Over that, and it doesn't come back to the pan fast enough.
As far as any pressure issues, they're fine up to when the problems start. If your clearances were fine on a stock motor, I would run 5w30 or 10w30. Just a rering and good clearances wouldn't dictate thicker oil. But verify that you have a good gauge as mentioned and a good filter (like Wix)-not Fram. Not sure about the delco, but the smaller one is probably hurting you here.
Did you verify the pickup to the pan clearance? If it's too low or too tight to the sloped up part of the pan sump, the pump will cavitate and not be able to pump oil at a certain rpm. In that case, though, the pressure usually comes back as soon as you let off the throttle.