I got a 77 impala with a 99 350 vortec, Its got a race cam, ported vortec heads, 8qt milodon pan with matching pickup and stock oil pump, a turbo 400 w/3500 stall and 4.10 gears, when I get the car on the highway around 70 mph my oil pressure goes from 40 to 20 psi if I slow dow it goes right back up. now at 70mph my rpms are around 3600 is this oil pressure drop normal?  Ive already replaced the pump and the motor doesn't burn oil, has good bearings and runs strong.  any advice?  

Try a different oil gauge.  

Does the oil pressure guage work from a sending unit or a pressure line from the block? If it's got an electronic sending unit, try changing it.  

Sounds like the sending unit for the guage might be bad.