Sounds like you checked all the mechanical concerns by checking the thermostat, gauge,and water pump ?
Now do this :
1. Water is alot better heat transfer agent then anti freeze. Make sure you only use about 30-35 % anti freeze in your cooling system.
2. You can use the 180 or 160 thermostat. I like the 180 better because of less cycling of the water between the radiator and engine. Gives the water more time in the radiator to cool.
Drill one 1/8" hole in the base of the thermostat. Position the hole so that its in the 12 o'clock position. The hole is to relieve air pres. buildup around the thermostat that could prevent it from opening at 180 or 160 degrees. Helps purge the air from the cooling system.
3. Most alum.heads are notorious for building up pockets of air around the # 8 and # 7 vave chambers. If the cooling system has been drained for whatever reason.
Upon refiling the system jack the front of the car up so that gravity will force the water back into the rear of the heads. Be sure and just add enough water so all the tubes in the radiator covered.About a half the tank ought to do it.
4 .With the cap off start the car while its still jacked up and let it idle for about 15 mins. slowly adding water as its needed in the tank. Be careful that the water from the tank doesent belch all over you.
5. Let the car down, fill your radiator full put the cap on ( at least 15 lbs. cap )and go for a nice easy drive for 6-10 miles. The gauge will go up to 230 or so and start to settle down as you drive.
Around the 180-200 range should be o.k. Ive seen alum.headed chevys run all day at 230 and never hurt them at all.
6. Back at the house let it cool down and add water again if needed.
Hope this helps