There is no place on the intake manifold for a oil sending unit. It's likely into the block, behind and underneath the dizzy; that's the stock location and can look like it's into the manifold.
I don't understand why your oil pressure light is on, could be the original wire is grounded somewhere - just a guess. If you pressure gauge is mechanical, tee off that port with the stock sending unit and you will have two methods to detect low oil pressure. If the gauge is electric, there is already a sending unit but it wont be compatible with the stock light. Check to see if someone has hooked it up this way, this might be the reason your light is on.
I don't know about the port by the oil filter, I've never seen a sending unit there - not saying it can't be done. Good luck.
79 Z28 bought new, honeymoon ride same year, still have same wife, same car.