I have been considering doing the same thing to my oil pump and don't know why it wouldn't work. If you look at LC Engineering's oil pressure adjuster, it looks like they drilled and tapped and installed a piece of rod.

You should be able to add a thin washer or two under the spring, but I would use thin ones and add them one at a time until you get the pressure you are looking for.

Why did you change oil pumps?

89 Ext cab 22RE 5 speed
84 Ext cab 22R 5 speed
88 Mitsubishi Macro Cab 2.6 5 speed