251 Oil PSI issues.

[ecpiano]

Chrysler flathead guys…I need some help. Here in the story for reference. 
Swapping a 251 into my M37. I found a bad main bearing and was able to source new bearings of the same size. Crank was in good shape and measurements were good. Put in new main bearings and reassembled. I needed a military oil pump as I am running military distributor. I primed the pump out of the engine and put it back in. I then reassembled the whole engine and cranked the engine with no power to the distributor and got oil pressure to my mechanical gauge. I then added power to the distributor and the engine fired right up. No oil pressure. I then removed the supply line to my remote oil filter (I have a full flow filter) and cranked with no power and oil flowed out. I put oil filter back on, cranked the engine with no power and got 20 pounds of pressure by cranking for a few seconds. I then started the engine and you can see the oil in pressure gauge line and the oil sucks back into the engine and no oil pressure.
 

I put new gauge on same as above. I took the oil filter off the filter housing and started engine. Oil came out of both holes. 
maybe my engine is missing a plug in the oil galley??

Anybody have any thoughts.

[D35 Torpedo]

Is your gauge plumbed into the correct port?

[kencombs]

Tell us a little more about the full flow oil system.   Is it a factory system with the block mounted filter or an add on.   Maybe some pics of the plumbing if it isn't factory.

[QEC]

You haven't told us what the pressure was like before the bearing swap.