No oil to rockers.

Oil flow in a /6 rocker system is just dribbles off the rockers, not a flow. It can take a minute or more of idling to see this coming out, especially up front. FYI, the oil feeds up through the very back rocker stand. Make sure the right rocker bolt is inserted in that hole; it is the long bolt. The oil has to flow around that bolt and up into the enlarged hole in the back mounting hole in the shaft that needs to face down on that rocker stand. Your shaft orientation described above is right.

Clearing a blockage in the oil feed hole could be challenging; it goes around one of the head bolts and up from the rear cam bearing. And the blockage could even be in the rear cam journal; there is an angled hole through the rear cam journal that allows oil flow about 5% of the time in pulses. So then engine has to be rotated by hand to find that spot that lines up the cam flow-through hole to clear it through. If blocked, I'd be trying some solvent down in there, followed by light oil after the solvent flows through, to water down the solvent in the oil system. Blowing through won't clear the passage through the cam journal unless it is lined up, and then the crud might go down into the rear main oil feed passage and block that up.

Unfortunately, you can't easily prime the oil system on a /6; the pump drives directly off the cam, not off an intermediate shaft like in a small block. Some guys have used a pressurized oil chamber feeding into the pressure relief port on the pump, but it takes some work. And to get it to the head, the cam has to be lined up just right.

If you want to try this operating to see if oil is coming up, then remove the back rocker bolt, turn the engine over by hand to see if the shaft flexes too much and if not, then fire it up . If the shaft flexes, I'd find a way to hold it down and watch down that hole.