So I guess we should start with oil pressure itself. With a freshly rebuilt engine at cold idle the oil pressure should be more than 40 psi. At hot idle, you should have some oil pressure on the gauge. If there is no oil pressure or very little at hot idle, then it wasn’t overhauled correctly, or the oil is not thick enough for the ambient temperature. The manual tells you what motor oil to use. With a slightly (or mostly) worn engine with oil pressure problems I have run 20w-50 in the summer.
From there you start looking at the oil tube coming from the main oil gallery up through the head to that little tee piece that goes to the rocker assembly. After the engine was run a few hundred miles were the rockers checked again for clearance? The oil shouldn’t be gushing from between the rockers unless they are badly worn, but with the cover off it should be enough to see some trickling down or splashing you from being flung out by actuating rockers as the engine runs at idle.
Sorry the picture is a bit fuzzy, but the only one I had on-hand. The feed tube and tee fittings should have a good seal. Hopefully the bottom of the tube is sealing properly as well. In the picture you can see the tube in the center of the engine between the middle pushrods and travel up to the tee. Hopefully this helps.