340 Oil Priming Question First Start

I would be surprised if the rocker shafts themselves are clogged. You could probe the mounting holes with electrical wire and carb cleaner to see if anything is in there. You can check the shaft exit oiling holes by removing the rockers and taking a look. You should carb cleaner the rockers thoroughly and inspect the oil exit holes for any clogs. Oil it all up and put it back together. It should oil.