You are not getting what I am saying.
You can modify and drill all you want. To get the oil correctly on the pushrod and ball, the oil hole in the rocker arm MUST line up with the oil hole in the shaft. It IS that simple. I have never seen an original 273 rocker shaft. I have only had DC or MP ones, and at one time I had some Isky shafts. Not one of them had the oil holes in the correct location.
Again, the ONLY way to get the correct oiling to the pushrod cup/rocker ball, the oil feed hole in the shaft MUST line up with the oil feed hole in the rocker.
This means that whoever is assembling the engine must, MUST mock the head up and physically LOOK to VERIFY the holes are lined up. This is with the rockers on the shaft, the valves in place at the correct installed height, and the adjuster removed and look at it. If you can't see the oil hole in the shaft you need to figure out why