Is there a reason you cannot cut into the OEM Instrument cluster you are using? Perhaps find a junk one and disect it to see how much room you have. Could then see if you can insert an OEM armature into that housing and incorporate that into your cluster.
Most 80's GM oil pressure is 0-90 ohm like the fuel gauge. (2 wires). Also volt would be 1 more wire.
It's possible you could use pointers from dissected cluster to he consistent. Just because you add armatures to a housing doesn't mean they cannot be wired independent of the cluster.
I've made many custom faces and took apart a good deal of 80's-02 assemblies. Not sure what unit you are using. Sometimes you can add to a panel. More work for sure but you get what you are after within reason.