Yes, intake manifold. There should be some kind of threaded hole somewhere in the intake. It will likely be plugged and may require some effort and heat to loosen it. I can't say about back when this engine mas made but most engines have had at least some drillable and tapable extra metal areas in the casting that can be opened up, even if they were not originally drilled and tapped.
If you would rather not mess with adding the VFF 30 you can always resort to adding an oil pressure switch to control the solenoid that is in use now.