Idling at a stoplight _should_ have you with more oil in the sump than any other time when the engine is running. Period. Even with only three quarts actually in the sump, it should be impossible for your car to uncover the pickup on initial launch -if everything is as it should be. An improperly located pickup or even the slightest crack in the tube could cause an instant loss of pressure.

If you had excess oil accumulation in the top of the engine, you would see a loss of pressure during extended high RPM operation, not immediately off idle.

I raced a 383 in '71-'72 with a stock pan, pickup and pump, shifted it at 6500 RPM and never had oil pressure issues. You have a problem somewhere. It will save you time and money to identify and fix it.