I am not sure what the port on top of the filter housing is used for. The oil pressure sending unit is in a difficult place to access on the driver's side of the engine near the ECM on the block. It is possible that the electrical connection to that sender may have been dislodged or damaged when you did your clutch job. I would definitely check out this area. Also, this is where you should test for oil pressure with the mechanical gauge.
If necessary, you can prove that you have oil flow by disabling the engine's ability to start by removing the fuel supply relay ( #51, I think) in the main fuse location in the engine compartment. Loosen the oil filter two or three turns and place a drain pan underneath. Have someone crank the engine for a few seconds (it should not start) while you verify that a steady supply of oil is coming from the filter area. You will probably set a check engine light, but it should reset after driving.
- John