To me another option is install a PRV to dump excessive water pressure and run the oil temp gauge. However looking at you old panel there appears to be room to run both by off setting the guages, also by relocating the ing. key and placing a gauge there.

There's nothing but room on that dash !

I have a hybrid glass electronic dash. I struggled with this as well.

I chose a tach and a water temp gauge.
(Ilmor has excellent oil temp control. )

I moved my secondary gauges above the cabin door.

You can back off hot oil and let it cool, but water temp off the chart indicates a restriction or a problem.

Pressure could be fine but you could still be overheating. Depend on where the sender is placed.

Without filling holes and doing glass work to re-arrange gauges, I don't see a ton of room available. I see maybe some room to the right of the helm, but its tight back there with the steering hoses.. and the wheel is going to be blocking the view in that area anyways.