Bill,
See pic below. That's the main supply oil passage from the pump...goes through a passage in the side cover. I drilled and threaded the middle section of the passage and installed a plug. This forces the oil out the first oil...though the cooler...back into the same passage through the second hole...then on to the oiling system as normal.
The oil pressure on these engines is not particularly high at all...but volume does seem to be ok if the flow is fairly unrestricted. The oil cooler offers little or no restriction..although there will be a pressure drop through the cooler...it's moot because the down stream restrictions are greater than that of the cooler...meaning the resistance to flow through the cooler is way less than that of the downstream system.
I have changed the cooler mounting to point the supply lines UP, this will keep the cooler full of oil during shut down and prevent having to refill the cooler on startup every time. I have no worries about the pump making enough pressure to make the climb to the oil cooler...the reason these engines have low oil pressure is not the pump...but the type of supply system they have...many places for oil to just flow out of bearings and into the sump...the cam bearings are probably the only place in the engine where there is enough restriction in flow to develop upstream oil pressure.
It gets complicated....but, believe it or not...I have a couple of years of schooling on hydraulics, hydro-dynamics, heat transfer and fluid flow. Which means I know enough to really screw things up

JohnnyB