If you've worked on engines you know how hot things can get. I think you're gonna need to get some antifreeze diluted 50/50 with water before you can make an honest assessment. How much water did you have to add after the engine overheated?
I haven't heard of many people having issues with a failed temperature gauge on the instrument cluster but it could be an issue. My gauge starts all the way to the left on a cold engine then ends up just right of center once car is at operating temp.

 

Check your cap on the coolant reservoir.

 

However, the water is still boiling despite the display cluster showing a completely acceptable engine temperature, which is my concern right now.

Thermostat opening temp is probably 200 degrees in your car. Pure water will easily boil. (your old chevy may have had a 160 or 180 degree thermostat)

With the appropriate antifreeze mixture and a good cap the boiling point is raised to about 260 degrees.

But - the "boiling" may be combustion gasses. Before spending any more money that you don't have - rent or borrow a combustion gas leak detector from your local auto parts store. That will tell you if you have a damaged head gasket or not - then you can plan your expenditures - rather than buy a cap and antifreeze and that does not solve your problem.

Modern temp gauges are dampened - to show normal temps unless drastically overheating. This keeps consumers happy - some tend to stress if the gauge move a tiny bit one way or the other.

 

with the engine cold, unscrew the coolant cap, is the coolant bottle under pressure, does it give a loud hiss ?
if yes its likely you blew the head gasket, confirm with block tester fluid.

Its not boiling off, its over-pressurizing from combustion gas.
Thats why the gauge reads normal.