I have an engine of unknown history . As far as I can tell, it was rebuilt at least the heads were changed but not ran since.

It spins freely by with decent resistance. I removed the oil cooler and valve cover, spun it a few times by hands ( at least 10 times) shouldn't I see oil coming out at least from the oil cooler holes ?

If it was never primed after the rebuild, it could take a couple of minutes of cranking (not all at once, the starter wont like that).

Take off the oil filter and see if you can get oil out there first.

I don't think you will see much oil movement by just turning the engine by hand.

you need to spin it a lot faster to get the oil to move, when we test fire engines it usually gets oil pressure after about four 20 second bursts with the starter. A starter usually spins and engine at roughly 200 RPM.