The compression test results are right on the edge of failing: There is a 15% maximum allowable difference between any two readings; and the 150/175 difference actually fails that standard if you take 15% of 150 (which is 22.5). A visual check for cracks or warpage is not adequate; the head must be tested by a machine shop that has crack testing equipment (magnaflux or zy-glo) and a level plate and dial indicator, or a surface grinder to take a light cut on the head to display any warpage. If the machine shop finds no cracks or warpage; the valves should probably be reground.
It is unfortunate that you went to all the work of pulling the heads; without first pressure checking the cooling system. Bubbles in the coolant are not always an indicator of bad head gaskets. One other cause of bubbles would be if this car has GM DexCool antifreeze in it, there is a known problem with that particular antifreeze formula attacking the silicone intake manifold gaskets which were used on many GM vehicles; and allowing air to leak into the cooling system. There was a huge class action lawsuit against GM because of that; which they got out of when they declared bankruptcy. I don't know whether this motor used silicone intake gaskets; but you should be able to find that out.
There is also an electric radiator fan on this vehicle; which is designed to run when the coolant temperature exceeds a preset value. This fan is activated by a radiator fan relay; which is known to be unreliable. If the fan has not been running; it will make the engine overheat. In that event, I would replace the radiator fan relay.