1997 Oldsmobile Cutlass Engine overheating

Engine Cooling problem
1997 Oldsmobile Cutlass 6 cyl Front Wheel Drive Automatic

I filled the surge tank when it was cold and started it up. I let it idle for 2 hours and the temperature gauge stayed either a little above or below the halfway mark.
I shut it off and let it cool for about a half an hour. The surge tank didn't appear to be filling back up so I refilled it.
I started it back up and left the cap off and the heater off.
As it heated back up the level in the surge tank came up. I put the cap back on and the coolant leaked out around the cap.

I have replaced the water pump and surge tank cap. I was told by a younger mechanic that the surge tank is building up 25 pounds of pressure and it shouldn't go over 15 pounds.

He said it was either the water pump, lower head gasket, or a cracked head he thinks it is leaking internally.

There is no coolant in the oil or coming out the exhaust. What else could it be.