What side clearance are you setting the rockers at? Too much clearance will also drop oil pressure. I shoot for 0.015 inches and make sure a 0.020 inch shim don't fit. I use a u shaped SS shim.

67 Coronet 500 9.610 @ 139.20 mph
67 Coronet 500 (street car) 14.82 @ 94 mph
69 GTX (clone) - build in progress......