From what I can see of your pictures, you have an existing "T" fitting at the oil pressure gauge port. One of those outlets would have fed the oil pressure gauge and the other the external line to the rockers, like on my 261 if I'm presuming correctly.
Before I go down the "unfortunately" road, did the center head bolt on the passenger side on your engine have a drilled hole up through the bottom and out the side of the threads? Here's a
'Bolter "Pre '68 Dave" Blog Post on the rocker oiling system subject and the "drilled" head bolt info that can better describe this than I can. No need to reinvent the wheel me thinks.
It's most likely (IMHO based on what's known so far) that with your oil ports behind the push rod cover plugged the same as mine, it's likely you will need to run the external oil supply line to feed oil to your rockers.
The only way I can think of starting to checking things further requires you to remove the head and look to see if the oil passage mentioned in Dave's post is blocked, plus look to see how the oiler that feeds from the head to the rockers is set up.
