A baffle in the front of the pan may help on decell. Does it already have a rear baffle? You are experiencing pressure loss on accell as well. This tells me the pan level could very well be low. Also 20 psi at idle with 20/50 shows it needs a fair amount of volume to make pressure at higher rpm. It could be bleeding excessive oil upstairs. Less internal leakage will leave more in the pan. Does it have 8.5 qts in the entire engine or just the pan? 8.5 qts in the pan should be more than enough with proper baffling. I can run mine with 6.5 qts in the pan with no fluctuation at either end. But it has a completely different pan. Baffles, swing trap door, swinging pick-up, and a full length diamond mesh tray.

Doug