the rockers are fed oil up from number 2 and 4 cam bearing where it then goes sideways a bout an inch to align with the hole going up to the bolt area for the rocker stand.

Blockage in these sideways passages always was a problem on the ford ohv engines from 1954 up thru the models until the 221 and 429 engines with the lifter/pushrod fed oil like the chevy.

Remove the rocker shaft and spin engine with starter to see if oil comes out those holes in the heads---if it does your rocker shafts may be assembled up side down (memory)

If not---u can try blowing down with air or even a u-joint grease gun tip on a grease gun to see if you can force out any sludge blockage--

If all else fails---remove heads to get at arre where the oil goes sideways