Re- torquing the head would be a logical step and if it stops the leak then problem solved. You would be able then to replace the valve cover gasket to make sure that isn't a possible leak location. If the leak persists then after 35 years then it's probably time to replace the gasket, check your valves and see if the head might be warped. How is the compression on the cylinder nearest the leak? All the cylinders?

There certainly are oil drain holes going through the head gasket. They are not pressurized but that doesn’t mean they can’t weep.
It certainly won’t hurt to try and retorque.
If you wind up replacing the gasket, be darned sure you clean out any liquid that gets into the holes so the bolts do not hydraulic lock before they actually clamp the head to the block.  Many a first timer has torqued the heads against a slug of coolant, then wondered why nothing works right any more. 
You might even pull the bolts one at a time and clean the holes out before retorquing in case that was what caused your problem in the first place.

Appears you could try re-torquing the head bolts.
 
This is from the 1966 Ford Service Manual for the 170-200 6-cyl engine:
“ … Oil is fed to rocker arm shaft assembly through a drilled passage in the cylinder block at the number 4 camshaft bearing.   The passage in the block indexes with a hole in the cylinder head.  The oil passage in the cylinder head is drilled from the cylinder head bolt bore to the No. 6 valve rocker arm shaft support. …”

The flow diagram included appears to indicate that the rear most head bolt is where this ‘passage’ is located.

MS - from the picture posted, and how the oil system works, seems to be a likely source.