It could be stopped up anywhere. I have seen the rocker shafts completely full of sludge.
You are probably on the right track. If you have good flow at the oil tube up to the head you can probably get by cleaning from there on. One thing: some of the later engines had a 1/4" dia rod about 3" long in the tube to limit the flow of oil. You may have to remove that tube and clean it out too.
If I recall correctly, the rocker shaft has plugs in each end held by cotter keys. I am thinking that I took a long rod and drove one plug completely through the shaft. After I had removed all the stands and rockers.
If you try this be careful: the other plug will come out like a bullet (one of mine got lost once and I had to make another.) and oil and goop wants to squirt out all the oil holes.