20W50 is going to be hard to pump around when cold or cool. Especially with the first 2 engines rebuilt with stock parts... wear in the oil pump can make it hard to even pick up cold 20W50. Coolant at start reaching 160F will still leave oil temps low; oil temps lag well behind water temps.
If your pumps are not spanking brand new, you need to change to using new pumps every time. And go with a lighter weight oil; if you are just drag racing, it is not all that hard on oil, not like circle track, road race, or rally where the engines are run hard for long periods of times. Drag racing CAN be hard in that the oil cools then heats and cools then heats, over and over; unless you are using a synthetic, the actual viscosity will vary a lot.
DO you have any oil pressure readings?